Expanse of sea in the Orkney Islands, Scotland, between Mainland and Hoy, until 1957 a base of the Royal Navy. It was the main base of the Grand Fleet during World War I and in 1919 was the scene of the scuttling of 71 surrendered German warships. It was abandoned as the main base for the fleet 1919 and operations transferred to Rosyth, but was reactivated as a base in World War II.