Scapa Flow, sea basin, northern Scotland, in the Orkney Islands. It is 24 km (15 mi) long, 13 km (8 mi) wide, and contains several islets. Scapa Flow was the principal naval base of the British during World War I and World War II. The German fleet that surrendered to the Allies in November 1918 was interned at Scapa Flow. On June 21, 1919, one week before the Versailles Treaty was signed, the German crew scuttled the German ships, preventing their subsequent use by the Allies. On October 14, 1939, during World War II, a German U-boat penetrated the supposedly secure harbor of Scapa Flow and sank the battleship HMS Royal Oak.