Scapa Flow is a large sheltered harbour located within the Orkney Islands. Because it is one of the largest natural harbours in Europe, it was strategically important in both world wars.

During World War I, it was a base for the British Grand Fleet, which set sail from there for the Battle of Jutland in 1916. After the armistice was signed, a large portion of the German Fleet was confined in Scapa Flow for many months. Eventually, German Admiral von Reuter gave the order to scuttle the fleet, and eight of the wrecks still lie on the bottom of the flow, where they attract SCUBA diving tourists.

The British Home Fleet was based in the flow during World War II, so that it could protect convoys in the North Atlantic. In 1939, German U-Boat U-47 managed to pass anti-submarine defenses and enter the flow, where it sank the battleship HMS Royal Oak, killing 833 men. Following this event, the Churchill Barriers were constructed.

Nowadays, Scapa Flow is used as a port for tankers to transfer oil, as well as being a popular destination for divers.