 One month into World War II , the Royal Navy battleship HMS Royal Oak was sunk by a German U @-@ boat in Scapa Flow . As a result barriers were built to close most of the access channels ; these had the additional advantage of creating causeways enabling travellers to go from island to island by road instead of being obliged to rely on ferries . The causeways were constructed by Italian prisoners of war , who also constructed the ornate Italian Chapel . The Scapa Flow base was run down after the war , eventually closing in 1957 . 
