U-218 was one of only six Type VIID submarines ever built - a mine-laying variant of the workhorse Type VII, stretched longer to accommodate five vertical mine tubes holding fifteen SMA mines apiece. At 76.90 metres long with a displacement of 965 tonnes surfaced, she was modest by U-boat standards, but the mine wells made her unique. Twin Germaniawerft diesels pushed her at 16 knots on the surface; submerged, two AEG electric motors crept her along at 7.3 knots. She carried twelve torpedoes for her five tubes, an 8.8 cm deck gun, and a complement of forty-four men. Her operational depth was 230 metres - deep enough to disappear, shallow enough that depth charges could still find her. Kapitänleutnant Richard Becker took command at her commissioning on 24 January 1942.

U-218's war reads like a catalogue of near-disasters and modest gains. On her first patrol in September 1942 she damaged the Norwegian Fjordaas, then was savaged by escorts of Convoy ON 127 the very next day and limped to Brest. Her second patrol ended after surface ships and aircraft mauled her south-west of Portugal. Her third and fourth patrols, in the summer of 1943, produced nothing. Then on 2 August 1943 a Vickers Wellington of No. 547 Squadron RAF caught her west of the Bay of Biscay - six wounded, the boat shredded, another emergency return. She had one bright moment off Martinique in November 1943, sinking the small sailing ship Beatrice Beck with her cargo of cod. A 86-day patrol the following winter produced nothing at all.

Her real work was the long, quiet violence of mine-laying. In October 1943 she dropped mines outside Port of Spain, Trinidad. In July 1944 a mine she had laid off Lands End damaged a British auxiliary cruiser badly enough to require repairs at Falmouth and Glasgow; the ship later returned to service as HMS Silvio. In April 1945, with Germany days from collapse, U-218 sank the Ethel Crawford in the Firth of Clyde with a mine laid forty-eight hours earlier. And there is a darker postscript - U-218 may have been responsible for the very last British ship lost as a consequence of the war. The steam fishing vessel Kurd struck a mine off Lizard Head on 10 July 1945, two months after victory in Europe. The mine was almost certainly hers.

U-218 surrendered in Bergen on 12 May 1945 and was sailed to Loch Ryan in Scotland to await Operation Deadlight, the Allied programme to scuttle Germany's surrendered submarine fleet rather than risk the technology falling into the wrong hands. Most boats were towed out into the Atlantic and sunk by gunfire or torpedoes. U-218 never made it that far. Under tow on 4 December 1945, she foundered north of Inishtrahull and slipped down on her own. In 2001 the marine archaeologist Innes McCartney located her wreck off Malin Head, finally fixing the position of a boat that had spent the war hiding from exactly this kind of attention.

What you are looking at, when you fly over this stretch of sea, is the world's largest U-boat graveyard. Operation Deadlight scuttled 116 boats here between November 1945 and February 1946 - some from Lisahally on the Foyle, some from Loch Ryan. Many lie within a few miles of U-218. The waters off Donegal and Inishowen are now an unmarked memorial to a technology that nearly choked Britain into submission. From cruising altitude, the sea looks identical to any other patch of North Atlantic. The bones below are arranged in a rough crescent following the towing routes from Scotland - hundreds of feet of submerged hardware, slowly becoming reef.