Scapa Flow is a body of water in the Scottish Orkney Islands. In response to the build-up of the Imperial German Navy's High Seas Fleet, Great Britain decided in 1904 that a northern base was needed to control the entrances to the North Sea. Scapa flow was chosen for the main base of the Royal Navy's Grand Fleet.
It was defensively reinforced, beginning with over sixty block ships sunk in the many entrance channels between the southern islands to facilitate the use of submarine nets and booms. These blocked approaches were backed by minefields, artillery, and concrete barriers.
Following the German defeat in 1918, 74 ships of the Imperial German Navy's High Seas Fleet were interned at Scapa Flow pending a decision on their future in the peace Treaty of Versailles. On 21 June 1919, after seven months of waiting, German Rear Admiral Ludwig von Reuter made the decision to scuttle the fleet.
Primarily because of its great distance from German airfields, Scapa Flow was again selected as the main British naval base during the Second World War. The strong defences built during the First World War had fallen into disrepair. On 14 October 1939, the German submarine U-47 penetrated Scapa Flow and sank the battleship HMS Royal Oak.
New blockships were sunk, booms and mines were placed over the main entrances, coast defence and anti-aircraft batteries were installed at crucial points. Winston Churchill ordered the construction of a series of causeways to block the eastern approaches. These "Churchill Barriers" now provide road access from Mainland to Burray and South Ronaldsay. After the war the Royal Navy left Scapa. The last facilities were closed down in 1957.