Scapa Flow, from Old Norse Skalpaflói, meaning "bay of the long isthmus" is a body of water in the Orkney Islands, Scotland, United Kingdom, sheltered by the islands of Mainland, Graemsay, Burray, South Ronaldsay and Hoy. Its sheltered waters have been used by ships since prehistory and it has played an important role in travel, trade and conflict throughout the centuries - especially during both World Wars.

The Harbour Authority area of Scapa Flow in Orkney has been measured as part of a wider consultation in ballast water management in 2013, and it has been accurately calculated that Scapa Flow is 324.5 sq km (125.3 sq mi) in area and that this area contains just under 1 billion cubic metres of water. It is currently a world-famous diving location, with the wrecks of the scuttled German Fleet offering unique diving challenges. Scapa Flow is also a major oil port serving the Flotta Oil Terminal, and is a prime location of ship-to-ship transfers of crude oil product and liquefied natural gas (LNG).

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