UB-22 was a Type UB II, the German Imperial Navy's coastal submarine - built fast, built small, built to slip out of the Flanders bases and harass shipping in the southern North Sea and English Channel. She was 36 metres long, displaced 263 tonnes surfaced, and could dive in 45 seconds. Two Koerting diesel engines pushed her at 9 knots on the surface. A single Siemens-Schuckert electric motor moved her at less than 6 knots submerged. She carried four torpedoes, fired from two 50-centimetre tubes, and a 5-centimetre deck gun for surface work. The crew was twenty-one men and two officers, packed into a hull where every centimetre of headroom had been argued over by naval architects.

UB-22 was ordered on 30 April 1915 and launched on 9 October the same year - the kind of construction pace only a wartime shipyard could maintain. Over eighteen patrols she sank twenty-seven vessels. Most were merchantmen carrying coal, food, and raw materials for the British war effort. The Type UB IIs were small enough to surprise lone freighters and quick enough to dive when destroyers appeared on the horizon. They were also vulnerable: thin-skinned, slow underwater, and almost helpless against a properly laid minefield. The Royal Navy had spent four years laying minefields across every route a U-boat might use to reach the Channel.

The exact minefield that killed UB-22 was British. The exact route she was taking that night cannot be reconstructed in detail - the boat went down with everyone aboard. What is known is that she was operating with a German torpedo boat as a surface escort, common practice for a submarine returning to base, and that both vessels detonated mines essentially simultaneously. There were no survivors from UB-22. Twenty-three men - twenty-one crew and two officers - drowned, or were killed by the explosion, or were trapped in a sinking hull as the lights went out. The North Sea closed over the wreck and kept it.

The phrase 'iron coffin' was German submariners' own, applied to their boats with grim affection long before historians borrowed it. By the end of the First World War, Germany had lost roughly half of all U-boats committed to the fight, and the majority of those losses were to mines and to the depth charges of destroyers - rarely to dramatic duels. UB-22's story is the typical one: a long string of successful attacks, then a single unseen explosion, then nothing. The wreck site - somewhere near 54.67 degrees north, 6.53 degrees east, in the German Bight - is a war grave under international maritime law. The crew remains aboard. Above them, North Sea wind farms now generate the electricity that powers Germany.