U-47 in Scapa Flow: The Sinking of HMS Royal Oak 1939

Even with Germany in the ascendency at the beginning of World War II, Scapa Flow was supposed to be the safe home base of the British Navy. Nothing and nobody could penetrate the defences of this bastion, which was built up to formidable levels in World War I and symbolized the faith placed by the British in the invulnerability of their navy. So how, in the dead of night on October 13, was Gunther Prien's U-47 able to slip through the line of protective warships undetected to sink the might...