Scapa Flow is a large natural harbour in the Orkney Islands archipelago, Scotland. It is surrounded by the islands of Mainland, Burray, South Ronaldsay, Swona, Flotta, Hoy and Graemsay. It became a base of the British Royal Navy and was used as such in both World Wars. In 1918, following the Armistice, the German High Seas Fleet was interned at Scapa Flow; on 21 June 1919 51 of the German vessels were scuttled by their crews. Many were later raised and sold for scrap, but those that remain are a popular site for divers. On 14 October 1939 the German U-boat U-47 entered Scapa Flow and sank the battleship Royal Oak with the loss of 833 lives. To guard against further incursions a series of causeways were built by Italian prisoners linking Mainland, Burray and South Ronaldsay, thus blocking the eastern entrances to the anchorage. A chapel built by the prisoners, using a converted Nissen Hut, has been restored and is a popular visitor attraction on Orkney.
The naval base at Scap Flow closed in 1956.
Stromness, on the island of Mainland at the western entrance to Scapa Flow, is a ferry port with links to Scrabster, near Thurso, on the Scottish mainland. Flotta, on the southern side of the Flow, has a major oil and gas terminal.