Warfare History Network
In October 1939, illuminated by the northern lights, the German submarine U-47 threaded its way through sunken barriers and slipped into the British anchorage at Scapa Flow, a 125.3-square-mile natural port off the northern coast of Scotland, in the Orkney Islands.
Penetrating the anchorage had long been an unrealized German dream, one that U-boats had attempted twice in World War I; both times they had failed. One U-boat had been rammed and run aground and the second destroyed with all hands.
http://nationalinterest.org/blog/the-buzz/1939-nazi-submarine-slipped-one-britains-biggest-bases-sunk-22935
