The Churchill Barriers are a series of four causeways with a total length of . They link the south of Mainland in the north to the island of South Ronaldsay via Burray and the two smaller islands of Lamb Holm and Glimps Holm. On 14 October 1939, the Royal Navy battleship HMS "Royal Oak" was sunk at her moorings within the natural harbour of Scapa Flow, by the German U-boat "U-47" under the command of Günther Prien. "U-47" had entered Scapa Flow through Holm Sound, one of several eastern entrances to Scapa Flow. To prevent further attacks, Winston Churchill ordered the construction of permanent barriers. They now serve as road links, carrying the A961. Work began in May 1940 and the barriers were completed in September 1944, but were not officially opened until 12 May 1945, four days after the end of World War II in Europe.