Anti-submarine indicator loop

An anti-submarine indicator loop was a submerged cable laid on the sea bed used to detect the passage of enemy submarines. Developed by the Royal Navy during World War I, they were extensively used by the Allies during World War II to protect harbours against submarine attack.

They worked as induction loops – the submarine's magnetism induced a current in the cable as the submarine passed across it. The technology was first developed at the Portland Naval Base in 1915 and the first operational use was at the Grand Fleet's anchorage at Scapa Flow. The German submarine UB-116 was detected by hydrophones at 21:21 on 28 October 1918 attempting to enter the flow via Hoxa Sound. Two hours later (at 23:32) current was detected in an indicator loop laid in a remotely controlled minefield, induced by the submarine at it passed over the cables. Mines around the loop were detonated by remote control, sinking the submarine. It was the last U-boat destroyed by enemy action before the Armistice.