Many Blockships were placed during WW1, after the German Submarine U-18 entered Scapa Flow. U-18 followed a steamer through the Hoxa Sound boom on 23 Nov 1914. Luckily a sharp eyed RN lookout spotted her periscope and she was rammed and sunk. On 28 Oct 1918 a second German submarine UB-116 was sunk by a remote-controlled mine, while trying to penetrate the defences of Scapa Flow. There is a useful little map on the floor of the Fossil Centre Museum in Burray. It shows the position and names of the Blockships and other wrecks in and around Scapa Flow.