U-155 was a Type IXC - the U-boat designed to operate not in the choke point of the North Atlantic but in the open ocean, the Caribbean, the South Atlantic, and the coastline of South America. She displaced 1,120 tonnes on the surface, stretched 76.76 metres bow to stern, and could travel 13,450 nautical miles at 10 knots without refuelling. That endurance is why she sank ships off Cape Hatteras, east of Barbados, west of Havana, and near the Cape Verde Islands. Her crew of forty-eight worked six torpedo tubes - four at the bow, two at the stern - and twenty-two torpedoes. She carried a 10.5 cm deck gun for cheaper kills and a complement of anti-aircraft guns that proved increasingly necessary as the war turned.

Her third patrol, in the summer of 1942, was U-155's most lethal. She sank ten ships totalling 43,514 GRT - tankers, cargo vessels, and a Brazilian liner called Piave, which she put down with the deck gun to save torpedoes. Off Cape Hatteras she killed Cranford in three minutes. Off Barbados she gave Barbacena a longer death. Off Sylvan Arrow she watched her victim sink eight days after the first torpedo struck - the salvage attempt had failed. Piening once treated two wounded survivors aboard U-155 before sending them back to their lifeboat with water, supplies, and a course to land. Then he sank the next ship he found. This was the schizophrenic morality of submarine warfare: ruthless beneath the periscope, sometimes humane on the surface, always returning to the same brutal arithmetic of tonnage.

By the summer of 1943 the war had turned. U-155's sixth patrol was bracketed with two other boats in the Bay of Biscay for mutual defence - four de Havilland Mosquito aircraft attacked on 14 June, three of them from No. 307 Polish Night Fighter Squadron. Five crewmen were wounded. The seventh patrol, north-east of the Cape Verde Islands, lasted weeks and found nothing. The eighth, off Brazil, sank just one ship before the Atlantic emptied of targets. The ninth patrol was 105 days long. Result: zero kills, two crewmen dead from Mosquito attacks on the return leg. The hunters had become prey. On 4 May 1944 U-155 shot down an RAF P-51 Mustang - a small triumph in a year that contained almost nothing else.

On 9 September 1944, U-155 left Lorient for the last time - the last U-boat ever to sail from that famous base before the Allied advance closed the bunkers. She returned to Germany by a circuitous route, docking at Flensburg on 21 October. There she sat out the rest of the war. Her commanders changed three times in those final months. For a brief period in autumn 1944 she had the youngest U-boat captain of the entire war: twenty-year-old Leutnant zur See Ludwig von Friedeburg. After the surrender the Royal Navy transferred her to Loch Ryan for Operation Deadlight. She was scuttled on 21 December 1945 in the waters off Donegal that became Germany's wartime graveyard.

In 2001 a team of divers led by the marine archaeologist Innes McCartney located her wreck off Malin Head, lying upright on the seabed at 73 metres, largely intact - a steel cathedral of dials and bunks and brass valves preserved in the cold dark. Six years earlier, in 1995, her crew had held their 25th reunion. Former Oberleutnant zur See Johannes Rudolph attended. So did one of the Mosquito pilots who had attacked them in June 1944. They sat in the same room, drank together, traded stories. The men who had tried to kill each other across the rolling deck of the Atlantic now compared notes - half a century older, both surprised to be alive. The ocean below this flight path holds a great deal.