The exhilaration of diving to a depth of 70 metres off the coast of Co Donegal to view the giant wreck of the Justicia is recounted by John McManus

Just before two o'clock in the afternoon on July 19th, 1918, a torpedo ploughed into the RMS Justicia as she passed Fannad Head, Co Donegal, bound for New York from Belfast

It was one of a pair fired by a German submarine, the UB-64, under the command of a Lieut Von Schrader. The torpedo stopped but failed to sink the 32,000-tonne luxury liner, which had been pressed into service as a troopship by the British Admiralty.

Such was the value of his potential prize that Von Schrader risked depth charges from escort vessels and stayed in the vicinity, attacking the crippled liner just over two hours later, this time firing another pair of torpedoes, both of which hit home

The Justicia was taken in tow by the tug Sonia which made for the relative safety of Lough Swilly. Von Schrader continued to shadow the stricken liner and made a third attempt to sink her just before 8 p.m. in the evening, firing one more torpedo, which again hit the target.

The U-boat commander made a fourth attempt later that night, firing another torpedo, but reported he could not see if it hit its target. Undeterred, he prepared to launch a fifth attack in the early hours of the following morning, but as he moved into position to attack, he saw two plumes of water as torpedoes fired from another submarine hit the liner.

The source of these two torpedoes is unclear. Two other German submarines were in the vicinity, according to accounts of the sinking recorded by Ian Wilson in his book Donegal Shipwrecks. The most likely candidate is the UB-124 which shortly afterwards surfaced accidentally, only to be sunk by three of the Justicia's escort party. Another candidate is U-54, under the command of Lieut Cmdr Von Ruckteschell, which was also in the locality.

Either way, the two torpedoes constituted a coupe de grace for the liner and she sank shortly after 10.30 a.m., though not before a skeleton crew had been taken off.

In total, only 16 of the ship's complement of 600 were lost. According to an eyewitness account from one of the Sonia's crew, she sank stern first while "the pressure of air escaping from the submerging hull blew off the ventilators and hissed from outlets until the bow finally disappeared so that nothing remained but some flotsam".

The Justicia now rests on the sea bed some 25 miles off Malin Head, Co Donegal. More than 85 years of winter storms blowing in from the North Atlantic have finished the job started by Von Schrader's torpedoes. The ship is now badly broken up, with her decks and bulkheads fallen in and her sides collapsed out onto the sea bed.

At a depth of 70 metres, she is beyond the limits of what could be considered safe for a diver breathing just compressed air and can only really be dived using a combination of oxygen, nitrogen and helium blended to minimise the potentially lethal effects of oxygen and nitrogen at that depth.

Decompression to avert the bends means that for 25 minutes spent on the bottom, a diver must spend another 60 minutes or so slowly making their way back to the surface with numerous stops for off-gassing on the way. To make this possible divers must carry four cylinders containing more than 9,500 litres of compressed gas.

Our first glimpse of the Justicia was as we made our way down a shot line leading from a buoy on the surface to the wreck. From a vantage point, some 30 metres above the sea bed, we get an appreciation of the size of the ship, with her wreckage spreading out, literally, as far as you can see in the 40-metre visibility afforded by the clear water off Donegal.

The Justicia was one of the largest merchant ships sunk in the First World War. The largest was the 48,000 tonne Britannic - sister ship of the Titanic - which was sunk by a mine in the Mediterranean in 1916. The other large loss was the Lusitania, also sunk by a German submarine in Irish waters, but off the Old Head of Kinsale. At 31,550 tons she was roughly the same size as the Justicia.

To get some sense on the size of these ships, consider that the Irish Ferries's car ferry, Isle of Inishmore - which has the capacity for 856 cars together with 2,200 passengers and is running between Rosslare and Pembroke this summer - has a gross registered tonnage of 34,000 tons.

The shared pedigree between the Justicia and the most famous of ocean liners, RMS Titanic, is clear from her bow section. It is one of the least broken up parts of the wreck and sits on the sea bed listing over slightly to one side, the safety rail at its apex - think Di Caprio and Winslet with arms outstretched in the James Cameron film - still in place.

The Justicia was the next ship laid down in Harland & Wolff after the Titanic was completed. Originally launched in 1914 as the Statendam, she was to be the flagship for the Holland America Line. But she was requisitioned by the Admiralty after the outbreak of the First World War and entered service as the Justicia in 1917. Intended for Cunard Line as a replacement for the Lusitania (hence the postfix "ia" on her name), she was instead allocated to the White Star Line because Cunard was unable to muster a crew for the vessel. White Star manned her with the surviving crew from the Britannic, thus completing the circle linking the three great liners lost in the war, and more tangentially the Titanic.

Although it is still bright at 70 metres in the clear water off Donegal, most of the colour has been filtered out, leaving the wreck cast in murky shades of green and grey.

Looking back from the bow towards the middle of the ship, we can see a long tube running off into the gloom, along what would have been the keel of the liner. This was a service tunnel and is big enough for us to enter through one of the numerous breaks, should we feel so inclined. Nobody does. Either side of the tunnel is a scrapyard of steel plates, girders and bulkheads, piled up on top of one another. Within view are a number of larger prices of wreckage, one of which, roughly the size of a modest suburban semi-detached, is what remains of the bridge superstructure. The lavish fit-out that was standard on great ocean liners of the time means that brass portholes and other ventilation gear are plentiful amongst the wreckage, but most are still firmly attached after 85 years on the sea bed and pay tribute to the quality of Harland & Wolff's ship building. And with only 20 minutes bottom time, the diver who stops to try and raise one is trading valuable exploration time for a lump of brass.

Off beyond the service tunnel, the first three of the Justicia's 12 boilers are visible, standing proud of the sea bed and the debris, looking for all the world like double decker buses, neatly parked at the depot.

Aft of them lies the stern and the ship's three massive propellers. But the size of the wreck - more than 750 feet long - is such that you could not see them and the bow in one dive.

Our time up, we head back to the shot line and make our way slowly to the surface, leaving the Justicia to her watery grave. Some 60 minutes later, we are finally back in the boat. The long hang in mid-water has done little to dampened the buzz of diving one of the world's best wrecks.