UC-42 was a Type UC II coastal minelayer, the most numerous German U-boat class of the First World War. The figures are tight and economical: 49.45 metres long, 5.22 metres wide, displacement of 400 tons on the surface and 480 submerged. Two six-cylinder four-stroke diesels gave her 520 horsepower for surface running and 11.7 knots; two electric motors managed 6.7 knots underwater. She could dive in 48 seconds and operate to 50 metres. Her offensive armament was unusual - six mine tubes in the bow holding eighteen UC 200 mines, plus three torpedo tubes (two forward, one aft) and seven torpedoes, plus the deck gun. Twenty-six men lived and fought inside that hull. UC-42 was ordered on 20 November 1915, launched on 21 September 1916 in Hamburg, and commissioned on 18 November 1916. She had ten months to do her work.

From 1 January 1917 onwards, UC-42 made six patrols from German-occupied Belgian ports. In total she sank fourteen merchant vessels totaling 9,877 gross register tons, and disabled a warship of 1,210 tons displacement. These were the numbers of the unrestricted submarine warfare campaign that Germany had begun on 1 February 1917 - a campaign that helped bring the United States into the war in April and nearly starved Britain in the spring. Each Type UC II laid mines on dark nights at the mouths of British and Irish harbours, then patrolled with torpedoes if shipping passed. The dirty arithmetic of the campaign accepted heavy U-boat losses in exchange for tonnage sunk. UC-42 was a small contributor by that brutal accounting. She was about to become one of the lost.

On 31 October 1917 the British torpedo boat TB 055 was escorting minesweepers at the entrance to Cork Harbour - the place we now call Roche's Point. At 1500 hours a crew member spotted an oil track on the water. TB 055 followed it, dropped a hydrophone, and heard 'hammering' and 'turbine-like noises' from below. Believing this was a submerged U-boat, the crew dropped a marker buoy and a depth charge. After the charge detonated, the volume of oil increased and bubbles rose. TB 055 signalled the armed trawler HMT Sarba for help. Sarba dropped another depth charge and held station overnight. The next morning HMD Sunshine and TB 058 swept the area. On 2 November, with oil still seeping up, dockyard divers descended and found a German U-boat lying on the seabed with her stern blown off. A brass plate on her conning tower read 'C42, 1916.' She was UC-42.

There is a quiet puzzle in the British Admiralty file. UC-42 had left Belgium on 1 September. The longest known cruise for a UC boat in home waters was 24 days. By 31 October, when TB 055 dropped its depth charge, UC-42 must already have been dead for weeks. So what were the 'hammering' and 'turbine-like noises' that TB 055 reported hearing? The most likely answer is that the submarine had already sunk - probably from one of her own mines exploding under her stern during a laying operation - and the sounds were currents moving through the wreck, or possibly imaginative interpretation by anxious operators on a wartime patrol. Some of the hatches on the wreck were found open. No survivors were ever recovered. Whatever happened underwater that autumn, it happened in silence and in the dark, sometime between Belgium and Cork.

The wreck was rediscovered on 6 November 2010 by Irish divers Ian Kelleher, Niall O'Regan, Philip Johnston, Eoin McGarry and Timmy Carey, in 27 metres of water off Roche's Point. The hull showed 'little obvious explosive damage' - inconsistent with a depth charge, consistent with the theory that she had sunk on her own mine. A serial number stamped on one propeller confirmed her identity. A second propeller, found 15 metres from the main wreck in 2022, was raised in 2024. The divers attached a commemorative plaque to the stern. Under International Maritime Law, UC-42 is now a designated war grave. She belongs technically to the Deutsche Marine - the modern German Navy. The 26 men inside her have lain together at the entrance to Cork Harbour for over a century, almost within sight of where the people of Cobh would walk every evening, unaware.