Scapa Flow (skăp´ə), area of water, 15 mi (24 km) long and 8 mi (12.9 km) wide, in the Orkney Islands, off N Scotland. It is bounded by the islands of Mainland, Graemsay, Burray, South Ronaldsay, and Hoy. Scapa Flow was Britain's main naval base in both world wars. Lyness, on Hoy, was the headquarters. The British vessel Vanguard exploded in Scapa Flow in July, 1917, and the German fleet was scuttled there in 1919. In Oct., 1939, a German submarine penetrated the area and sank the Royal Oak, causing the British fleet to withdraw until 1940. The Churchill Barrier was begun the same year to block the eastern entrance to Scapa Flow by sinking 250,000 tons of concrete in the sounds linking Mainland, Burray, South Ronaldsay, and two smaller eastern islands. The barrier now forms a causeway linking Mainland to South Ronaldsay. The naval base was closed in 1956.