A set of four causeways created from blocks of concrete and stone, the Churchill Barriers were designed to protect the eastern approaches to Scapa Flow in the Orkney isles. The barriers, extending to approximately 1½ miles (2.5 km) in length, cross four sounds, linking Mainland Orkney with Lamb Holm, Glimps Holm, Burray and South Ronaldsay. The building of the barriers was prompted during World War II when a German U-boat penetrated the Flow in October 1939 and sank the British battleship Royal Oak with the loss of more than 800 lives. Shortly afterwards, on the orders of Winston Churchill (1874 - 1965) who was then First Lord of the Admiralty, plans were drawn up for a series of causeways topped by a roadway which would effectively block the channels. Construction began in 1940 and the barriers were completed in 1945. Around 66,000 huge blocks of concrete were laid on a foundation of more than quarter of a million tonnes of rock, the blocks preventing the tide from sweeping over the road.