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5 September 1914 – the first warship sunk by a torpedo fired by a submarine

The British scout cruiser HMS Pathfinder had the misfortune of being the first ship sunk by a locomotive torpedo fired from a submarine in warfare. To be clear, she was not the first ship sunk by a submarine in combat – which, as history records, was the 1,240-ton American Civil War three-masted screw sloop USS Housatonic. She was blockading the Confederate-held port of Charleston when, on the night of 17 February 1864, she was attacked by the Confederate submarine H. L. Hunley. The small 40ft-long Hunley carried a crew of eight: seven to turn the hand-cranked propeller and an officer to navigate and steer. The Hunley made a stealth approach just under the surface towards the Union ship, and although she was spotted on her final approach, was able to ram a spar torpedo attached to her bow into the starboard beam of the Union ship. The Hunley then withdrew, and the torpedo exploded and sent the Housatonic to the bottom. Then the Hunley herself sank with the loss of all hands, for unknown reasons, shortly after the attack.

The 2,940-ton scout cruiser HMS Pathfinder – the first warship to be sunk by a torpedo fired by a submarine.

HMS Pathfinder was launched on 16 July 1904 at Cammell Laird’s yard in Birkenhead, on the River Mersey at Liverpool. (Cammell Laird is one of the most famous names in British shipping, and a massive, vibrant industrial firm today.) After fitting out afloat, Pathfinder was completed on 18 July 1905.

She was to be the lead ship of the Pathfinder class of four pairs of scout cruisers. Scout cruisers were smaller, faster and more lightly armed than armoured cruisers and light cruisers. They were intended to range far ahead of the fleet, as the name suggests, scouting for the enemy but not engaging heavier vessels. A second group of seven scout cruisers was ordered under the 1907–1910 government shipbuilding programmes; these would be more heavily armed. Scout cruisers were however an evolutionary dead end, and although all these ships served during World War I, they quickly became obsolete as faster and more heavily armed classes of destroyers and light cruisers were developed.

Pathfinder displaced 2,940 tons fully loaded and was 385 feet long overall, with a beam of 38 feet 9 inches and a deep load draught of 15 feet 2 inches. She was driven by two screws that were powered by two 4-cylinder triple expansion engines – steam being generated by 12 water tube boilers. This gave her a top speed of 25 knots, fast at the time of her construction – but by the beginning of World War I, the new classes of light cruisers, destroyers and torpedo boats could make 27 knots. The scout cruisers were only marginally quicker than the battleships they were meant to scout for, which could make 21 knots, and the scout cruisers could be matched in speed by battlecruisers.

The three-funnel scout cruisers such as Pathfinder were also intended to operate as the lead ships of destroyer flotillas – but it was found in practice that the scout cruisers had poor range. They only carried 160 tons of bunker coal to feed their 12 boilers and power their two 4-cylinder triple expansion engines. With a limited range, and now being outrun by the newer classes of destroyers and light cruisers, they were relegated to secondary duties.

Scout cruisers were lightly armoured, with variable waterline amour thicknesses on different parts of the hull. Pathfinder had 2-inch-thick vertical armour covering her engine rooms, but the armour did not run the full length of her hull. She had a partial armoured deck ranging from 1.5 inches to 5/8 inch thick. Her conning tower had 3-inch armour.

When she was built, she was fitted out with ten quick firing (QF) 12-pounder guns and eight QF 3-pounder Hotchkiss light naval guns – and as was common with warships of this period, she was fitted with two submerged 18-inch torpedo tubes. Two further QF 12-pounder guns were subsequently fitted and the eight QF 3-pounder guns were replaced with six heavier 6-pounder guns.

In 1911–1912, in the run-up to World War I, her original but by now outdated 12-pounder guns were replaced by nine more powerful faster-loading QF 4-inch guns. A brand new design introduced in 1911, the new QF 4-inch light naval gun would become standard on most Royal Navy and British Empire destroyers during World War I.

Pathfinder spent the early part of her career with the Royal Navy Atlantic Fleet, before being transferred to the Channel Fleet, and then to the Home Fleet. As the opening shots of World War I were fired, she was attached to the 8th Destroyer Flotilla, based at Rosyth in the Firth of Forth.

Great Britain declared war on Germany on 4 August 1914, and Germany quickly scored some notable successes against Royal Navy warships with the laying of sea mines. HMS Amphion, one of the second Active-class group of improved scout cruisers, was sunk by a mine laid by the German auxiliary minelayer SMS Königin Luise, just two days into the war on 6 August 1914, off the Thames Estuary. The mine broke Amphion’s back and caused her forward magazine to explode with the loss of 132 crew. On 3 September 1914, the old torpedo gunboat HMS Speedy, built in 1893, and now converted into a minesweeper, hit a mine and sank in the North Sea, 30 miles off the Humber, whilst attempting to assist the minesweeper HMS Linsdell, a victim of the same minefield. But although there were losses to mines in the first month of the war, there had been no loss to a torpedo, and the Royal Navy did not fully understand, or accept, the threat to surface vessels from submarines.

As the war began, Britain began a naval blockade of Germany, intended to cut off her maritime war supplies and to prevent the Imperial German Navy from breaking out into the North Sea and Atlantic to attack British shipping. The only way Germany could blockade or interdict British supply shipping was by the new submarine weapon.

The German Navy was inferior to the Royal Navy in numbers of ships – and so, to reduce the numerical inferiority, Germany embarked on a submarine offensive intended to sink as many British warships as possible and even the playing field. Ten submarines were initially sent out to attack Royal Navy vessels.

The submarine campaign however did not go well for Germany at first. During the first six weeks of the war, two of her submarines were lost for little or no return in British shipping. But this was all about to change – and Pathfinder would have the misfortune of being the first British warship to be sunk by a locomotive torpedo during this submarine offensive.

The U 19-class submarine U 21 of III Flotilla and two other German submarines were tasked to raid British naval units in and around the Firth of Forth, where the major British naval base at Rosyth was established. The Firth of Forth is a wide expanse of water on the east coast of Scotland: Edinburgh and North Berwick sit on the southern shores of its narrower section as it opens out to the North Sea.

U 21 was under the command of the 29-year-old Leutnant zur See Otto Hersing, who would go on to sink 40 Allied ships, totalling almost 114,000 tons of shipping, as well as damaging two others. (After later sinking the pre-dreadnought British battleship HMS Triumph on 25 May 1915 and then the pre-dreadnought battleship HMS Majestic off Gallipoli two days later, on 27 May 1915, he became known as the Destroyer of Battleships by his colleagues. He would go on to survive the war, passing away in 1960.)

At the beginning of September, whilst approaching the Forth Rail Bridge in the Firth of Forth, the periscope of U 21 was spotted near the Carlingnose Battery, which opened fire without success. Hersing withdrew U 21 from the Forth and commenced a patrol in safer waters, from May Island southwards.

On the bright sunny morning of 5 September 1914, Hersing spotted HMS Pathfinder heading south-south-east, followed by elements of the 8th Destroyer Flotilla. At midday, the destroyers came about and began to head back towards May Island. Hersing watched as Pathfinder detached from them and continued her patrol to the south.

Later that afternoon, whilst at periscope depth, Hersing again spotted Pathfinder – this time she was returning to her base. With her endurance limited by her poor stocks of bunker coal, she was only making about 5 knots, to conserve her coal. A speeding warship was a very difficult target for any submarine to hit, but at this lumbering slow speed she presented a relatively easy and valuable target for Hersing.

At approximately 3.45 p.m., Hersing gave the command for a single torpedo to be fired.

Lookouts on Pathfinder spotted the torpedo track heading towards their starboard bow at a range of 2,000 yards. The officer of the watch, Lieutenant-Commander Favell, gave orders for the starboard engine to be put astern and the port engine to be set at full ahead with full helm. This should turn her bow to starboard as quickly as possible and allow her to comb the track of the torpedo, and avoid it.

The attempt to comb the track of the torpedo however failed – she was most likely unable to manoeuvre quickly enough given her initial slow speed of 5 knots. The torpedo closed at speed and then hit her just forward of the bridge. It is suspected that the torpedo blast may have ignited the silk bags of cordite propellant charges for Pathfinder’s main battery guns and caused a flash, because there followed a second, massive explosion within the fore section of the ship, as the forward magazine blew up. Any crew below decks in the forward section were killed instantly. The foremast and No 1 funnel collapsed and toppled over the side.

The bow section of the ship, on the other side of the explosion, sheared off and sank like a stone. Pathfinder gave a heavy lurch forward and immediately took on an angle down by the bow of about 40 degrees. Water came swirling up the ship and quickly began to envelop the bridge and searchlight platform. The command was given to abandon ship, but the stricken ship was going down by the bow so quickly that there was no time to swing out the lifeboats.

As the water-filled forward part of the ship sank quickly into the sea, the stern lifted up out of the water and a massive pall of smoke rose into the air. Although the huge explosion in Pathfinder had happened well within sight of land and should have been seen and heard, in an effort to attract attention as she settled into the water, her captain ordered the stern gun to be fired. The gun mount perhaps had been damaged by the force of the explosion, because after firing a single round, the gun recoiled and toppled off its mounting. It rolled over the quarterdeck and then went over the stern, taking the gun crew with it. A short time later, the ship disappeared from sight below the surface, taking most of her crew down with her.

One of the few survivors later recounted how he had been below deck when the explosion occurred. He quickly got himself up on deck, only to slide down the sloping deck and become jammed beneath a gun. He was carried underwater as the ship went down but managed to free himself and swim to the surface.

Fishing boats from the port of Eyemouth were first to arrive at the scene of the disaster, only to find an expanse of sea that was littered with the scattered debris of a ship’s passing and a slick of fuel oil. Clothing, bodies and parts of bodies floated on the surface amidst the debris.

In the distance, the two-funnel 350-ton destroyer HMS Stag and the 465-ton torpedo boat destroyer HMS Express both observed the plume of smoke from the explosion – and each capable of making 30 knots, they turned to steam for the scene. It is said that as one of the destroyers arrived on scene it had an engine problem, which turned out to have been caused by a dismembered leg in a sea boot blocking a seawater intake.

There were only 18 confirmed survivors from Pathfinder’s crew.

At first, the British authorities attempted to cover up the true cause of the sinking, fearing to reveal just how vulnerable to torpedo attack British warships were. The loss of Pathfinder was therefore at first reported as being caused by a mine, the Admiralty having already reached an agreement with the Press Bureau that allowed for wartime censoring of all reports in the national interest.

Nevertheless, newspapers began to publish eyewitness accounts reflecting what had really happened, such as that of an Eyemouth fisherman who had assisted in the rescue, who confirmed rumours that a submarine had been responsible. The true story eventually came out, and the sinking of Pathfinder by a submarine made both sides in the conflict aware of the potential vulnerability of large ships to attack by submarines.

If further confirmation of the killing power of torpedoes fired from a submarine was needed, it came just a few weeks later. Early on the morning of 22 September 1914 in the North Sea, the three 12,000-ton Cressy-class cruisers Aboukir, Hogue and Cressy were sunk by a single submarine, U 9, under the command of Kapitänleutnant Otto Weddigen.

U 9 of I Flotilla had been tasked to patrol and attack British shipping at Ostend. At about 0600 on 22 September, U 9 spotted the three patrolling British cruisers and closed on her first target, Aboukir. U 9 then fired a torpedo from about 500 metres, which struck the British cruiser on the starboard side, flooding the engine room and causing the ship to slew to a stop.

The two cruisers Hogue and Cressy, initially believing that Aboukir had struck a mine, closed the stricken ship to rescue survivors. U 9 then fired two torpedoes at Hogue from a distance of about 300 metres. Both torpedoes were hits – she was mortally wounded, and capsized and sank within 10 minutes.

Shortly after, U 9 fired two torpedoes at Cressy from her stern tubes at a range of just under 1,000 metres. One torpedo missed – but the other hit the cruiser on her starboard side. U 9 then came about and fired her remaining bow torpedo at Cressy, striking her in the port beam. Cressy heeled over and capsized.

In this one action, three valuable 12,000-ton British armoured cruisers had been sent to the bottom of the North Sea, with the loss of 1,459 officers and men. Coming so soon after Pathfinder, it was another stunning success for the German submarine campaign.

The following month, on 15 October, the same submarine, U 9, sank the 7,770-ton protected cruiser HMS Hawke in the North Sea whilst on patrol off Aberdeen. Then the pre-dreadnought battleship HMS Formidable was torpedoed and sunk in the English Channel on 1 January 1915 by U 24. In all nine Royal Navy vessels had been sunk in the opening months of the war for the loss of five German submarines. If the German submarine threat had not been fully understood and feared by the Royal Navy at the beginning of the war, it certainly was now.

The hand-thrown Lance bomb.

At the beginning of World War I, the Royal Navy had no effective means of detecting a submerged submarine and could only rely on physically sighting the periscope or its wake – and then firing on the periscope with their guns. Early anti-submarine weapons were rudimentary, like the hand-thrown Lance bomb, essentially a grenade on a stick that was hurled down by hand when the vessel was physically above or beside the submarine.

In the run-up to World War I, Britain had feared that foreign authorities might not allow its merchant ships to enter port if they were armed. But as the German submarine threat began to materialise, Britain began to arm its merchant ships with a single stern gun, equivalent to what a submarine might carry as a deck gun. Civilian captains were encouraged to use their greater speed to flee a surfaced submarine and shoot back from their more stable gun platform.

The first British merchant ship lost to a German submarine was the 866-ton British steamer SS Glitra, which was stopped by U 17 on 20 October 1914. In accordance with international maritime law, her crew were given time to launch their lifeboats and abandon ship before she was sunk. This pattern of giving crews time to abandon ship would prevail until the beginning of the following year.

On 5 February 1915, Germany published a formal Notice declaring all waters around Great Britain and Ireland a war zone. Then on 18 February 1915, she began a campaign of unrestricted submarine warfare within that zone against merchant ships: any shipping, including that of neutral countries, would be sunk without warning and without regard for the lives of the civilian crews. German submarines began to sink an average of 100,000 tons of shipping per month.

Unrestricted submarine warfare continued until September 1915, when it was temporarily abandoned after an international wave of condemnation and the intervention of U.S. President Woodrow Wilson, following the sinking of RMS Lusitania on 7 May 1915 and other ships carrying American civilians.

♦ ♦ ♦

HMS Pathfinder was sunk by a torpedo fired from U 21 on 5 September 1914 and by a subsequent secondary explosion. Her wreck now lies in 64 metres of water in the Firth of Forth, off the Scottish east coast. Her damaged bow section sheared off and now lies about a mile away.

The wreck of HMS Pathfinder was known in the 1970s to fishermen as a fastener or snag for their nets – and when she began to be dived in the 1980s, she was reported as sitting upright, festooned with nets. In the 1990s as the wreck began to be visited more easily by divers, ropes were still hanging from her lifeboat davits.

Pathfinder sits on an even keel in 64 metres of water in a deep channel in the middle of the Firth of Forth, which is so wide here that the land seems very distant – you feel almost as if you are in open water. She is in a depth that is well within the modern technical diving range. So I determined to dive her and see this fascinating piece of naval history for myself.

My regular dive buddy, Paul Haynes, and I booked ourselves onto a technical dive boat that runs out of Eyemouth, and with a fully laden jeep filled with two full sets of technical diving rig and two underwater scooters (diver propulsion vehicles – DPVs) we drove the three hours down from Stonehaven on a Friday evening to stay overnight locally in Eyemouth and be ready for an early ropes off the next morning to catch slack water – the Holy Grail of diving, the time when the tide would go slack on the wreck and there would be no current to fight against.

The next morning, we were up early for a full breakfast – I always like to stock up well first thing when I am going to be out at sea all day. Next came the laborious task of ferrying all our dive kit, rebreathers, weights, bailout cylinders and scooters along the jetty and onto the dive boat.

Finally, after working up a bit of a sweat, it was done, and it was time for ropes off. Our skipper skilfully took the boat away from the jetty and we moved slowly north-east out of the quaint, ancient fishing harbour. As we left the harbour, the mainland was to our left and local skerries to our right. To our north and east lay the North Sea.

Once in open water, we turned to the north-west and began to motor up towards the Firth of Forth – towards the last resting place of Pathfinder. It was a warm calm day, the early morning sunlight sparkling off the blue water and casting long shadows. We passed the rocky foreshore and cliffs of the famous St Abb’s Head National Nature Reserve on our port beam, before leaving the land behind as we headed out into the open expanse of the Firth of Forth. Our destination lay far offshore.

As we neared the site, I popped into the wheelhouse and watched the echo sounder as the boat slowed on our approach to the site. On the first pass, the familiar multi-coloured silhouette of a wreck far below, rising a good 5–10 metres off the seabed, appeared on the sounder. We were in business – and the crew readied the shotline, a weighted line with a large buoy at its other end.

The UK has semi-diurnal tides, which means that the seawater flows in one direction, say south, for roughly six hours, before turning to move in the opposite direction, north, for another six hours. The current gets progressively stronger from the beginning of the six-hour period until midway through the cycle, after which the strength of the current begins to drop away and lessen towards the point when the tide begins to turn, at which point the water goes slack; there is little or no tidal flow.

The actual strength of the tide at any one time in the cycle depends on celestial mechanics and the alignment of the sun, earth and moon. There are two types of tides - the stronger are called spring tides, whilst the weaker are called neap tides.

Spring tides: When the sun, moon and earth are in a line and so there is a new or full moon, the gravitational pull of the sun on the earth’s water adds to the gravitational pull on the water by the moon. This causes the water on the earth to bulge outwards towards the sun and moon. As the earth rotates, the bulge, locked towards the sun and moon, appears to sweep around the earth in the form of a long-period wave. We get the highest high tides and the lowest low tides, and the tidal flow each way is strong. Spring tides are nothing to do with spring or the seasons – they occur naturally twice each lunar month, all year long.

Neap tides: When the sun and the moon are at right angles to each other respective to the earth, the bulge of the ocean caused by the sun is partially cancelled out by the bulge of the ocean caused by the moon. We get weaker neap tides – with lower high tides and higher low tides. The tidal flow, and its rise and fall, are not as extreme as with spring tides – and just like spring tides, neap tides occur twice a lunar month, all year long.

At the moment when the tide turns to run in the opposite direction, the current, which relentlessly ebbs and floods in these six-hour cycles in UK waters, drops away and lessens to almost nothing as it swings around to begin to move in the opposite direction for the next six hours. It’s the magical time called slack water.

Divers in tidal waters always aim to arrive on site well before slack water to give time to shot the wreck by dropping a line with a heavy weight tied to one end that has a buoy on the other end to keep it afloat. Sufficient time is always allowed for divers to get kitted up, everything being timed so that the water is just going slack as you enter the water to begin your dive. (In diving we say that you can never be too early for slack water. It will always come – but if you are too late for slack water, slack water won’t come around for another six hours.) Divers descend down the shotline, often called the downline, and then for safety, at the end of a deep decompression dive or a dive in an exposed location, will tend to return to the downline to ascend.

The DSMB fully inflated on its reel. It is common to write the diver’s name on the very top (which will protrude above the water) in large letters so those on the dive boat can identify who is below. © Bob Anderson

Delayed surface marker buoy (DSMB) rigged for diving. It is clipped or stored somewhere convenient on the dive rig.

In the north-east of Scotland we get slack water of about 20 minutes at springs – and almost two hours at neaps. So, if we are diving a wreck on a spring tide, we aim to get that precious slack water whilst we are down on the wreck itself, in the knowledge that as we begin our ascent the tide will have turned and the current will be picking up.

But in the North Sea at springs we can get currents of 1–1.5 knots, and it is not feasible for a group of divers on ascent to all try to hang onto the shotline down to the wreck for perhaps an hour of decompression: it would be a rough hour with the water whipping past you at about one knot. (A knot – one nautical mile per hour – may not seem very fast, but when you’re immersed in that water its force is considerable.)

As a result, technical divers ascending from moderate depths often carry out a free ascent, hanging on a reel under their red 6-foot-tall sausage-like delayed surface marker buoy (DSMB), which is inflated and sent to the surface as they ascend so that topside know where they are – in an hour of decompression in UK waters, divers will drift perhaps half a mile or more away from the dive boat. When the skipper of the dive boat sees DSMBs coming up, he knows to leave the fixed downline and shadow the DSMBs until the divers break the surface.

The alternative way of doing this sort of free decompression ascent in tidal waters is to deploy a free-drifting decompression station. This can be a decompression trapeze, or at its simplest, a weighted line, both of which get carabinered to the downline at 20–30 metres and have their own big surface buoy(s).

The trapeze is simply three long aluminium tubes that are horizontally secured to vertical ropes at either end of them, the tubes being positioned at depths of 12, 9 and 6 metres. The ropes at either side are tied off to their own large buoys, which suspend the whole contraption.

Either way, the trapeze or separate weighted buoy line can be laced with spare bailout cylinders of breathing gas at different depths to make sure everyone has enough gas if there is a problem. As rebreather divers, we all carry our own bailout cylinders under our arms, which hold sufficient breathing gas for us to do the whole dive open circuit if the rebreather malfunctions and we have to bail out off it onto our spare cylinders. So, in theory, no one should need any gas. But the unexpected often happens … as divers we say you can never have too much gas underwater. But you can have too little – and then you are in big trouble.

The practice is that the trapeze is carabinered to the downline at a suitable depth with a transfer line – that is, a line that allows divers to transfer from the downline to the trapeze.

As the last diver comes up from depth at the end of the dive, when they arrive at the point where the trapeze or deco station is carabinered to the downline, the transfer line can be unclipped from the fixed downline. Everyone then goes for a drift, holding on to one of the trapeze bars. Drifting with the current in a fixed body of water, you now feel that you are stationary in the water – whereas, in reality, you are speeding over the seabed far below at anything up to a knot.

My group has a tag system to assist in knowing where everyone is. On the way down the shotline, at the beginning of the dive, we clip a plastic tag with our name on it onto a fixed ring on the shotline beside the trapeze carabiner that is to be unclipped to allow us to drift and ascend. On the way back up, each diver removes their name tag from the ring – so if your tag is the last one on the ring, you know everyone else is above you and that it is safe to unclip the trapeze and go drifting.

Like most technical divers, we also have a system that only red DSMBs are fired up on ascent if all is well. This tells topside boat cover that all is good. We also each carry a yellow DSMB and reel, which is only deployed to the surface to tell topside that there is a problem.

As a result of the area that Pathfinder lies in, although the underwater visibility in the shallows above the wreck can be quite good, the silty seabed can be stirred up as the tide runs over the seabed, and it is common to find that down on the wreck the particles in suspension filter out all light coming down from above. As a result, there is little or no ambient light – the wreck usually has the feeling of being very dark and moody. Divers are reliant on their torches, the rusty red brown metal of the ship being covered in the soft coral known as dead men’s fingers, which flares white in the torch beams.

For UK technical diving on wrecks like Pathfinder, where you expect it to be pitch black with often poor, silty visibility in torch beams, each diver also carries a small strobe which is clipped to the downline a few metres above the wreck. The downline itself would be next to impossible to find without it, and doing a free ascent from great depth on a tidal wreck which is known to have many nets snagged on it is not the best idea. But 5–6 strobes flashing away in the darkness can be seen from a long way off. By the end of the bottom time, a diver’s night vision will have kicked in and you often see a fuzzy halo of light from the strobes flashing well in the distance.

On this visit, the skipper having positioned his boat to take account of the tide, he then gave the command for the shotline to be deployed over the side of the boat, intentionally placing the shot on the seabed just off the wreck. Skippers are very sensitive to not dropping weighted shotlines on war graves – particularly on fully munitioned warships like Pathfinder.

Our group of divers had dressed into our drysuits some time before on the approach to the site; pee valves (or should I say, offboard urination devices) were already all connected up. With the wreck shotted and slack water approaching, we began to wriggle into our rebreather harness webbing, pulling on fins and mask, clipping on bailout cylinders under each arm, connecting suit inflation direct feeds and switching on our rebreather wrist computers to let them start going through their boot-up self-check menus. Finally, fully rigged, we simply sat still carrying out our rebreather final pre-breathe for a few minutes – if there is going to be an early problem with a rebreather, it’s better it happens on the boat than in the water. All was good, we were ready to dive.

The skipper asked if Paul and I, being the most experienced, (what he meant, I suspect, was the oldest) if we would splash first and make sure the shot was near the wreck. We heavily stood up from the kitting-up benches on the dive deck and in the rather clumsy, ungainly gait of a fully rigged technical diver, carefully clumped our way over to the dive gate through the stern gunwale. At a signal from the skipper, it was one stride forward and we were splashing heavily into the water.

Righting myself, I dumped air from my buoyancy wings and drysuit and started to sink slowly. As the water closed over my head, I looked around and was surprised at how good the underwater visibility was. After an OK signal with Paul we started the descent down the line in about 20-metre visibility.

My optimism for such good visibility down on the wreck was abruptly smashed at about 40 metres down, when the water started to get rapidly murkier. By 50 metres down it was a silty brown with only about 5 metres visibility. This was most likely the result of the trawling in the channel that had been taking place up-current earlier.

We pressed on down into the gloom, our torches struggling to punch through it. At about 60 metres, the seabed began to materialise a few metres beneath me, at 64 metres. I shone my powerful torch around, up against the gentle current, and there at the limit of my vision was a brooding dark mass that seemed to be ominously rising up above me. Or at least that’s what I thought I was seeing – most divers looking for a wreck in dark conditions are familiar with the feeling of thinking there’s a dark silhouette out there, which recedes as you approach it; it’s just an illusion.

We clipped a reel onto the downline and reeled out as we moved across what turned out to be a gap of 5–8 metres until we arrived at a solid wall of rusted steel. We had arrived at Pathfinder’s starboard side, the hull disappearing down into the silty seabed. Shining my torch up the hull plating here, I could see where the wall of steel ended at the horizontal main deck above me.

We rose up this vertical wall of steel until we were able to pop over the bulwark onto the main deck at just under 60 metres, and here we tied off the reel line. The other divers wouldn’t need to go all the way to the seabed and rack up unnecessary deco – they would just come down to the reel and then move straight across to the main deck. I looked up the downline and thought I could see the faintest trace of their torches far above us as they descended.

We appeared to have arrived halfway along the ship, between the bridge and the stern. We moved off slowly on our scooters, forward along the starboard side of the hull, past the open circles where her three smokestacks had stood on top of a slender superstructure that was one deck level high. Dotted along the starboard side of the deck were lifeboat davits – some of these still with the original ropes hanging from them despite more than 90 years on the bottom.

There is a pronounced rise at the back of the bridge superstructure: the hull rises up a deck level to the fo’c’sle deck and two rows of portholes were studded along the side of the ship here. The stump of the foremast rose up, directly abaft the bridge. It had been brought down by the force of the explosion in 1914, along with the foremost smokestack.

Moving up on top of the remaining bridge superstructure, we made out the circular outline of the conning tower. This wreck is a military war grave and British divers have shown great respect for it over the years; there has been no pilfering of artefacts that I am aware of. As a result, small personal items were still strewn about here in the bridge area – I spotted a brass sextant and brass cage lights and lanterns.

A number of brass 4-inch shell cartridges littered the ship here, and immediately beside the empty grooved circular mounts of her 4-inch guns, a number of non-ferrous boxes were stacked side by side. Each box still held six ready-use 4-inch shells – the circular bases of some of the shells had corroded away to expose the rods of spaghetti-like cordite propellant inside.

I left the bridge area and moved further forward and downwards, into the gloom. The fo’c’sle deck seemed to begin to slope downwards abruptly – and then it just ended, sheared clean across by the secondary magazine explosion. It looks as though the ship heaved upwards as the massive explosion blew the bow off, bending the leading edge here over and downwards.

Ancient large gauge heavy netting was snagged over the break. This may have been old commercial fishing net – or something more poignantly related to the loss. For after the sinking, the Royal Navy put a net over the vessel to catch bodies floating out of the ship. This was a common practice with Royal Navy vessels, and one that would be repeated during World War II with, for example, the sinking of the battleship HMS Royal Oak at Scapa Flow in 1939.

Paul and I turned the dive here at the sheared-off fo’c’sle deck. There was no point venturing out into free water here – we knew the bow section was missing and lay almost a mile away.

We moved aft down the port side of the wreck, past more empty lifeboat davits, the three funnel openings and the skeletal one-storey deckhouse from which they rose. As we moved aft we began to see the torch beams of the other divers moving here and there like light sabres, the divers themselves invisible in the darkness.

As we got to the very stern we found more scattered 4-inch shells beside an empty 4-inch gun mount. Was this the mount for the 4-inch gun which had been fired after the torpedo hit and had gone over the side taking its crew with it?

Moving round the stern, I shone my torch downwards and could see the three-bladed starboard prop in free water where the tide had created a scour pit round the stern of the ship. I traced the free section of shaft forward from its support bearing until it disappeared into its hull tube and forward to the engine room.

After 25 minutes exploring her remains, Paul and I called the dive and began to scooter back to the downline to ascend. The downline was easily found off the starboard side – a number of strobes were flashing away on it 5 metres off the wreck in the gloom. We retrieved our reel and wound in our line as we moved towards the downline and began to ascend.

As we rose above 50 metres, our surroundings began to get brighter again. We were rising out of the cocoon of darkness that shields Pathfinder. Then, at about 40 metres we seemed to pop out of the cloud of silty gloom into bright water. We suddenly had 20–30-metre visibility again.

We reached the transfer line and moved slowly across it towards the trapeze that we could see hanging in the water high above us. As we rose we started slowly going through our decompression stops, all the time moving towards the trapeze. As we got shallower, every now and then one of the other divers would appear from the gloom far below us, carefully carrying out their own deco stops.

Finally, the last diver up disconnected the transfer line and we all began to drift under the trapeze, moving slowly upwards as we carried out our deco stops at 12 and 9 metres before the long hang at the last stop, 6 metres. As it was getting a bit busy on the trapeze, Paul and I came off the trapeze and whiled away the deco time circling the other divers on our scooters.

Here at the end of the first dive, it is perhaps the right time to explain, in case you’re new to this, a little about breathing gases and decompression, to start breaking you in gently!

The open-circuit (OC) divers in our group arriving at the deco station were breathing from standard diving regulators, where, as you breathe out, your exhaled breath is vented as bubbles from your regulator that rise up to the surface.

As the dive was deeper than the safe recommended limit for diving standard compressed air, they were using a helium-rich breathing gas for the deep part of the dive, known as bottom mix. As they ascended at the end of the dive and began their decompression stops at about 20 metres or shallower, they were able to switch over to a cylinder of enriched air nitrox (EAN) slung under their arm on their webbing and designed purely for use during decompression; it is called deco mix. Perhaps it is best if I also explain a little about diving gases and accelerated decompression at this early stage.

Basically, the more oxygen in your deco mix, the more you can shorten – or accelerate – your decompression stops. But there are certain depth limitations for different levels of oxygen in your deco mix, as these increased oxygen levels when you are diving can be dangerous at different depths.

The air you are breathing just now, reading this book on the surface, is comprised of 79 per cent nitrogen and about 21 per cent oxygen. Although largely inert on the surface, at high pressure levels nitrogen has a narcotic effect – the nasty diving problem called nitrogen narcosis. So both of the elements that make up ordinary air, nitrogen and oxygen, can become problematic when you are diving deep.

Nitrogen narcosis is a creeping (and at first largely unnoticeable) debilitating effect, which starts for me (when I’m diving on air) at a depth of about 30–35 metres. You need to know a little about the mechanics of diving to understand how it becomes a problem.

As a diver descends, the increasing weight of water surrounding them tries to compress internal air spaces such as their lungs, which are, simplistically, just bags of air. Imagine taking an air-filled crisp bag down underwater – it would very quickly be compressed to a fraction of its size by the surrounding water pressure. To avoid this eventually fatal effect happening to a diver’s lungs, an aqualung (or breathing regulator) delivers increasing amounts of compressed air with each breath as they descend. The aqualung delicately and rather cleverly keeps the air pressure in the diver’s lungs exactly equal to the increasing water pressure around the diver. The lungs stay the same size as topside, and no catastrophic collapse happens.

Once a diver has descended to a depth of 10 metres, the weight of the surrounding water in which they are immersed is conveniently exactly equal to the weight of the whole atmosphere that presses down upon us whilst we are standing on land at sea level. On the surface, the weight of the atmosphere (atmospheric pressure) is called one atmosphere or one bar. So, adding the 1 atmosphere weight of the atmosphere itself to the 1 atmosphere weight of water at 10 metres produces a pressure (water pressure) of 2 bar (or 2 atmospheres): at 10 metres, the water pressure is exactly double the air pressure we experience on the surface. The doubled weight of water and atmosphere above the diver will compress the volume of any air spaces such as lungs to half its normal size if an aqualung is not used.

To combat this ‘squeeze’ as the old hard-hat divers called it, at a depth of 10 metres a diver’s aqualung feeds them air at twice atmospheric pressure, that is at 2 bar. The delicate equilibrium between the air pressure in the lungs and the surrounding water pressure is maintained.

At a depth of 40 metres the water pressure is five times atmospheric pressure – that is, 5 bar – and comprises the 1 bar (atmosphere) on the surface plus 1 bar (atmosphere) for each of the four 10 metres. Any air spaces such as lungs would be compressed to a fifth of the volume they would be on the surface – not good. So, the aqualung again cleverly feeds a diver air that is compressed to five times atmospheric pressure – 5 bar. Again, the air pressure in the diver’s lungs is kept exactly the same as the surrounding water pressure – and the diver’s lungs remain exactly the same size as on the surface.

Boyle’s Law – the law of inverse proportions – governs this effect. When scientists were trying to work out what happened to air underwater, some brave, hardy men would sit in an upturned barrel cut in half, which was lowered into the water. As the barrel was taken below to predetermined depths, the air inside was compressed and the water level rose. Marks would be made on the side of the barrel at different depths. The depths and compression marks were correlated, and the law became clear.

If each breath the diver takes holds five times as much air as normal (compressed into the same volume), the diver is absorbing five times as much of the individual constituents. Therefore, in every breath the diver breathes in five times as much nitrogen, and five times as much oxygen.

Nitrogen is largely inert on the surface; the 79 per cent of nitrogen you are breathing right now as you read this is passing in and out of your body harmlessly. But the deeper you go, the higher are the volumes of compressed air breathed in each breath – and the more the increasing amounts of nitrogen in your body start to cause the debilitating effect known as nitrogen narcosis. Cousteau with typical flair called this effect the ‘Raptures of the Depths’.

For me, breathing air at 50 metres is roughly the same as drinking four pints of beer. The narcosis strips away your ability to understand and rationalise situations – and robs you of the ability to deal with things when they go wrong. The ‘narcs’, as they are affectionately known, affect people in different ways. Some get euphoric – some get paranoid. Some people get tunnel vision; others lose control and panic when the simplest thing goes wrong – something that could easily be dealt with normally by the same person on the surface.

Rather than breathing compressed air at any depth greater than 40 metres, nowadays I always dive on a trimix diluent, which replaces a large element of the dangerous nitrogen in the breathing mix with helium, which has no discernable narcotic effect. Although nitrogen narcosis is no longer an issue for me, if you want to get an idea of what nitrogen narcosis can do, I recounted a hit I got in the chapter entitled ‘Bail out on the Cushendall’ in The Darkness Below. This was a 58-metre air dive in 3-metre visibility, in a current, on the wreck of the World War II casualty SS Cushendall which lies off north-east Scotland. Oh, the things we do when we are young …

Whereas oxygen is very therapeutic and beneficial in normal use, as the aqualung feeds a diver increased volumes of breathing gas on descending, this means that in addition to getting higher partial pressures (or concentrations) of nitrogen, the diver also gets increased volumes of oxygen.

Oxygen, the very stuff that keeps us alive on the surface (and of course underwater as well) becomes increasingly toxic in the larger volumes breathed by divers as they venture deeper. The risk of an oxygen toxicity hit becomes a very real danger. This starts off with twitching and spasms but rapidly develops to uncontrollable convulsions where a diver will amongst other things, rip off their mask and spit out the breathing regulator. In water, a hit nearly always results in drowning, unless the diver is wearing a full-face mask. A number of leading technical divers have sadly ‘ox-toxed’ over the years and died of the uncontrollable convulsions – so deadly underwater. Some had mistakenly breathed their shallow water oxygen-rich decompression gases at too great a depth, quickly bringing on a fatal oxygen toxicity hit.

The trick is to use a nitrox mix which has the right amount of oxygen to safely accelerate decompression – and to use it at the right depth where the nitrox does not become toxic. The consequences of getting it wrong can be fatal.

A commonly used enriched air nitrox (EAN) mix that is suitable to breathe and shorten decompression (compared to breathing standard compressed air all the way to the surface) from a depth of 20 metres upwards is EAN50, which comprises 50 per cent oxygen and 50 per cent nitrogen. The increased amount of oxygen and reduced amount of dangerous nitrogen shortens (or accelerates) the time needed for decompression before surfacing.

At a depth of 20 metres, the water pressure on your body is three times what the atmospheric pressure on your body is as you read this right now. So the aqualung feeds the diver three times as much compressed breathing gas to keep the pressure in the lungs exactly the same as the surrounding water pressure – and avoid a lung collapse. This means that in every breath the diver is breathing in three times as much oxygen as on the surface. If each breath is 50 per cent oxygen, or half of the total mix, we could say that at the surface that the partial pressure of oxygen (abbreviated to PO2) is 0.5. At 20 metres, breathing three times as much oxygen the partial pressure is 3 x 0.50 = PO2 of 1.5 bar.

Trials have shown that a PO2 of 1.4 is relatively safe, but above a PO2 of 1.6, you are entering an area where the oxygen concentration in your body is starting to become toxic – and if the levels increase or if that same level is breathed for more than a certain time, you risk an oxygen toxicity hit, convulsions and death. That’s why we put a maximum depth limit on breathing EAN50 of 20 metres, where the PO2 is 1.5 bar.

But EAN50 has a fixed percentage of oxygen in it at all times – 50 per cent. Thus, at 10 metres, the PO2 is twice 0.5 = 1.0 bar. There’s less therapeutic oxygen in the breathing mix compared to breathing, say, EAN80 with 80 per cent oxygen, where the PO2 is 1.6 bar. So, although EAN50 is good because you can start breathing it deeper, at 20 metres, and start reducing the level of nitrogen in your body early, in the shallows it is not giving you as much oxygen as you could safely breathe. You can breathe pure 100 per cent oxygen from 9 metres to the surface, which is extremely good at accelerating decompression. Thus, in the shallows EAN50 is not such an effective decompression gas.

Many divers do in fact use EAN80 (80 per cent oxygen and 20 per cent nitrogen) for decompression. This higher oxygen level is very beneficial for decompression, but can only be breathed from about 12 metres up to the surface. To breathe EAN80 deeper than 12 metres or to breathe EAN50 deeper than 20 metres, or EAN100 (100 per cent pure oxygen) deeper than 9 metres, risks a potentially fatal oxygen toxicity hit. Thus, every deco mix, be it EAN50, EAN80, EAN100 or whatever, all have their own depth limits where the amount of oxygen in the mix becomes toxic – and potentially fatal.

In a contrast to open-circuit diving, Paul and I, in common with the majority of technical divers, have for a long time been using closed-circuit rebreathers (CCRs).

Whereas in open-circuit diving the exhaled breathing gas is vented to the surface, a closed- circuit rebreather continuously recirculates the same breathing gas – there is no venting to the surface. One of the benefits of using a rebreather is that a diver can program their onboard computer to never let the PO2 in the breathing gas loop drop below a certain level.

As a diver rebreathes through a CCR, during each breathing cycle (that is, one inhale and one exhale) the diver’s body metabolises some of the oxygen as it passes through the body, producing carbon dioxide (CO2). The expired breathing gas thus contains less oxygen than the gas the diver inhaled. In a CCR, that expired gas is cleaned of the dangerous CO2 in a scrubber canister filled with sofnalime and then analysed in a chamber in the rebreather by onboard oxygen sensors. The results trigger a solenoid switch to open and bleed just the right amount of oxygen into the breathing gas to keep the PO2 at the desired level of say 1.3 bar, no matter what depth the diver is at. So, on the ascent, all the way to the surface, the rebreather is trying to inject oxygen to keep the PO2 at say, 1.3 bar. At 20 metres, a CCR diver can be breathing a PO2 of 1.3 bar – but in contrast to breathing EAN50 on open circuit, at the final deco stop at 6 metres a CCR diver is breathing almost pure oxygen. The diver is getting the optimum amount of oxygen, so beneficial to decompression, at any point.

A modern rebreather, the A.P. Diving Inspiration Vision, popular with technical divers. The corrugated hose leading from the mouthpiece over the diver’s right shoulder is the ‘exhale’ hose of the breathing loop. The corrugated hose leading over the left shoulder to the mouthpiece is the ‘inhale’ hose. The wrist-mounted computer handset is in the foreground. © Bob Anderson

Rear view of a popular closed circuit rebreather (CCR).

The diver’s exhaled breath moves from the mouth through the exhale hose that runs over the right shoulder and into the bottom of the central stack between the two cylinders. From there the exhaled breathing gas passes upwards through a canister holding the ‘scrubber’ sofnalime material, which strips the dangerous carbon dioxide out of the exhaled breathing gas.

After passing through the scrubber, the cleaned, exhaled gas passes into a chamber at the top of the stack, where three or more oxygen sensors analyse the resulting breathing gas to determine how much oxygen the diver has metabolised in the last breathing cycle. Two electronic controllers (essentially minicomputers) then trigger a solenoid (switch) that injects the correct amount of high-pressure oxygen into the breathing mix to raise the depleted oxygen level back up to the desired level (the PO2 ‘setpoint’).

The cleaned, analysed and adjusted breathing gas then passes through the inhale hose that runs over the diver’s left shoulder directly to the mouthpiece, and the breathing cycle repeats. No breathing gas is vented to the surface – it is continuously cleaned, analysed and corrected as it is rebreathed.

The right-hand cylinder holds high-pressure 100% oxygen. The left cylinder holds the ‘diluent’, the desired breathing gas – air or trimix. The small black cylinder on the left holds the diver’s drysuit inflation gas. © Bob Anderson.

Of course, too much oxygen is also a problem. If something goes wrong, say the solenoid switch sticks open and the oxygen level in your breathing gas goes above that set level, audible alarms go off and red lights blink on the heads-up display (HUD) unit that is usually mounted on the corrugated hose breathing tube just below and off to one side of your mask in your peripheral vision. Too much or too little oxygen, and the normally green lights start flashing red warnings.

I generally use a PO2 of 1.3 bar, but with deep repeat dives I back it off to 1.1 bar just to stop racking up too high levels of oxygen over a period of days. I usually manually inject oxygen in my final decompression stops to keep the PO2 at 1.4 bar and shorten decompression.

The wrist-mounted computer handset on my Inspiration Vision CCR. The top left figure reads 0.70 and confirms the pre-set PO2 set point being used. The three figures –in this picture all at0.81 – show the individual readings of the three oxygen monitoring cells that continuously analyse the breathing gas. The readings should be roughly consistent – if one figure differs wildly from the other two then it is an indicator that the cell is possibly malfunctioning.

The horizontal white rectangle at the top is the scrubber monitor: it displays how the scrubber in the back-mounted canister is performing. © Bob Anderson

A fully rigged technical diver with a shallow bailout nitrox cylinder slung beneath the right arm on the ‘oxygen’ side. In this case the cylinder holds EAN50, which has a maximum breathing depth of 20 metres – and this is clearly marked on the cylinder, to avoid the wrong gas being breathed at the wrong depth, a standard tek diving practice. To breatheEAN 50 deeper than 20 metres for a prolonged time risks an oxygen toxicity hit with possible fatal convulsions. The mask strap is under the hood,to avoid it being kicked or knocked off.© Bob Anderson