Scapa Flow is a natural harbour surrounded by the Scottish Orkney Islands. Because of the build-up of the German Imperial Navy's High Seas Fleet, the British navy decided it would be the home of their Grand Fleet before the outbreak of the First World War. Despite its strategic importance it was not a safe anchorage. It was vulnerable because of bad weather, strong tides and the many entrances. Therefore coastal batteries, minefields and anti-submarine obstacles were constructed to defend the fleet. Also blockships were sunk to seal the many entrances to the harbour and prevent access to enemy ships.
The First World War proved that Scapa Flow was a good choice. In 1916, the Grand Fleet intercepted an outbreak of the German High Seas Fleet. The following Battle of Jutland became the greatest naval battle in history. The outcome of the battle was indecisive but the High Seas Fleet would never leave their harbour again, so the British Royal Navy maintained their superiority. After the war the British fleet left Scapa Flow.
From 1933, the political situation in Europe was getting worse with the rise of NAZI Germany. Scapa Flow became once again the home of the British fleet. At the beginning of the Second World War the old defences had fallen in disrepair. On 14 October 1939 the German U-boat U-47 penetrated Scapa Flow and sank the battleship HMS Royal Oak. 833 of the crew were lost. After this incident it became one of the most strongly fortified harbours in the world.