Scapa Flow (Old Norse: Skalpaflói) is a body of water in the Orkney Islands, Scotland, United Kingdom, sheltered by the islands of Mainland, Graemsay, Burray, South Ronaldsay and Hoy. At 140 square miles, with a sandy bottom, and relatively shallow (not deeper than 160 feet, and most of it about 70 feet deep), it is one of the great natural harbours/anchorages of the world, with sufficient space to hold a number of navies. Viking ships anchored in Scapa Flow more than 1000 years ago, but it is best known as the site of the United Kingdom's chief naval base during World War I and World War II. The base was closed in 1956.
John Rushworth Jellicoe, admiral of the Grand Fleet, was constantly nervous about potential submarine or destroyer attacks on Scapa Flow, and the base was reinforced with minefields, artillery, and concrete barriers starting in 1914. These fears were borne out when German U-boats twice attacked British ships in Scapa Flow, though the attacks themselves did no damage. The first, by U-18, took place in November 1914; but the sub was rammed by a trawler searching for submarines while it was trying to enter Scapa Flow, causing the submarine to flee and then sink. The second attack, by UB-116, in October 1918, encountered the sophisticated defenses then in place at Scapa Flow, was detected by hydrophones and then destroyed by shore-triggered mines before it could enter the anchorage.
Three days after this submarine attack, four Luftwaffe Junkers Ju 88 bombers raided Scapa Flow in one of the first bombing attacks on Britain during the war. The attack badly damaged the elderly base ship, the former battleship HMS Iron Duke, but one bomber was shot down by an anti-aircraft battery on Hoy.
The visitor centre is situated on the island of Hoy and is situated in the former naval fuel pumping station and a converted storage tank. The exhibits include a large three dimensional representation of the island and of the ships as they were prior to scuttling. The island is accessible by local ferry several times daily. The centre has catering facilities for day trippers.
The wreckage of the German fleet has become increasingly popular as a venue for recreational Scuba divers. Divers must first obtain a permit from the Island Harbour Authorities, which is available through diving shops and centres. The wrecks are mostly located at depths of 35 to 50 metres. Decompression is pretty much essential if divers wish to explore these wrecks properly, while the shallower dives will benefit from nitrox. Divers are permitted to enter inside the wrecks, but not to retrieve artifacts located within 100 metres of any wreck. However time and tide has washed broken pieces of ships' pottery and glass bottles into shallow waters and onto beaches. The underwater visibility, which can vary between 2 and 20 meters, is not sufficient to view all the length of most wrecks at once, however, current technology is now allowing 3D images of them to be seen
The wrecks of the Royal Oak and the dreadnought Vanguard, which exploded at anchor during the First World War (1917), are considered war graves. Only Royal Navy divers are permitted to visit them.