Built to defend against German submarines, the Churchill Barriers are causeways linking what Scottish group of islands?

The Churchill Barriers are a series of four causeways in the Orkney Islands, United Kingdom, with a total length of 1.5 miles (2.3 km). They link the Orkney Mainland in the north to the island of South Ronaldsay via Burray and the two smaller islands of Lamb Holm and Glimps Holm.

The barriers were built in the 1940s primarily as naval defences to protect the anchorage at Scapa Flow, but now serve as road links, carrying the A961 road from Kirkwall to Burwick.

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.