Scapa Flow attack on the HMS Royal Oak, by the U 47 of Captain Günther Prien.
Scapa Flow, meaning “bay of the long isthmus” i s a body of water in the Orkney Islands, Scotland, United Kingdom, sheltered by the islands of Mainland, Graemsay, Burray, South Ronaldsay and Hoy. The Harbour Authority area of Scapa Flow in Orkney has been measured as part of a wider consultation in ballast water management in 2013, and it has been accurately calculated that Scapa Flow is 324.5 square kilometres (125.3 sq mi) in area and that this area contains just under 1 billion cubic metres of water. 
This statistic makes Scapa Flow the second largest natural harbour in the World after Sydney Harbour, Australia. Scapa Flow is one of Britain’s most historic stretches of water – located within the Orkney Islands, off the northeast coast of Scotland. Its sheltered waters have been used by ships since prehistory and it has played an important role in travel, trade and conflict throughout the centuries – especially during both World Wars.
Primarily because of its great distance from German airfields, Scapa Flow was again selected as the main British naval base during WWII.
The strong defences built during WWI had fallen into disrepair. Defence against air attack was inadequate and block ships sunk to stop U-boats from penetrating had largely collapsed. While there were anti-submarine nets in place over the three main entrances, they were made only of single-stranded looped wire, there was also a severe lack of the patrolling destroyers and other anti-submarine craft that had previously been available; efforts began belatedly to repair peacetime neglect but were not completed in time to prevent a successful penetration by enemy forces.
On 14 October 1939, under the command of Günther Prien, U-47 .jpg)
penetrated Scapa Flow and sank the WWI–era battleship HMS Royal Ark anchored in Scapa Bay. After firing its first torpedo, the submarine turned to make its escape; but, upon realising that there was no immediate threat from surface vessels, it returned for another attack. The second torpedo blew a 30-foot (9.1 m) hole in the Royal Oak, which flooded and quickly capsized. Of the 1,400-man crew, 833 were lost. The wreck is now a protected war grave. Three days after this submarine attack, four Luftwaffe Junkers JU 88 bombers raided Scapa Flow in one of the first bombing attacks on Britain during the war. The attack badly damaged an old base ship, the battleship HMS Iron Duke, with one bomber shot down by an anti-aircraft battery on Hoy.
New blockships were sunk, booms and mines were placed over the main entrances, coast defence and anti-aircraft batteries were installed at crucial points, and Winston Churchill ordered the construction of a series of causeways to block the eastern approaches to Scapa Flow; they were built by Italian prisoners of war held in Orkney. These “Churchill Barriers” now provide road access from the mainland to Burray and South Ronaldsay, but block maritime traffic. An air base, RAF Grimsetter (which later became HMS Robin), was built and commissioned in 1940.
The death of Günther Prien, age 33.
U-47 left Lorient (France) for her tenth patrol on 20 February 1941. Only four days later they attacked convoy OB-290 and sank four ships totalling 16,310 tons. The last radio message from U-47 was received on the morning of 7 March, giving a position south of Iceland in the North Atlantic.
It had been long supposed that U-47 with the whole crew, 45, was sunk on 8 March 1941.
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