At the outset of World War II, Scapa Flow was supposed to be the safe home base of the British Navy – nothing could penetrate the defences of this bastion. So how, in the dead of night, was Gunther Prien's U-47 able to slip through the line of protective warships to sink the mighty Royal Oak? This book provides the answer with an account of one of the most daring naval raids in history. Drawing on the latest underwater archaeological research, this study explains how Prien and his crew navigated the North Sea and Kirk Sound to land a devastating blow to the British. It reveals the level of disrepair that Scapa Flow had fallen into, and delves into the conspiracy theories surrounding the event, including an alleged cover-up by the then First Sea Lord, Winston Churchill.