On 14 October 1939, the Royal Navy battleship HMS Royal
Oak was sunk at her moorings within the natural harbour
of Scapa Flow in a night-time attack by the German U-boat
U-47 under the command of Günther Prien.  In response,
First Lord of the Admiralty Winston Churchill ordered
the construction of several permanent barriers to prevent
any further attacks.  Work began in May 1940 and was
completed by September 1944.  However the barriers
were not officially opened until 12 May 1945, four
days after the end of World War II in Europe.