German submarine U-47 sank the battleship HMS Royal Oak at Scapa Flow on 14 October 1939.
Commanded by Günther Prien, U-47 entered the British naval base at Scapa Flow through a passage that had been obstructed by blockships. It fired torpedoes at Royal Oak, and a later salvo struck the battleship, which capsized and sank.
The attack killed 833 people, including Rear Admiral Henry Blagrove. Royal Oak was an old Revenge-class battleship, but its loss inside a major British anchorage had a significant psychological and operational effect.
U-47 became one of the most famous U-boats of World War II. It disappeared in March 1941 during a patrol in the North Atlantic, and its exact wreck location and cause of loss remain uncertain.