In this fashion the submerged hulls were made into air-tight chambers and raised with compressed air, still inverted, back to the surface. Cox endured bad luck and frequent fierce storms which often ruined his work, swamping and re-sinking ships which had just been raised. At one stage, during the General Strike of 1926, the salvage operation was about to grind to a halt due to a lack of coal to feed the many boilers for the water pumps and generators. Cox ordered that the abundant fuel bunkers of the sunken (but only partly submerged) battlecruiser be broken into to extract the coal with mechanical grabs, allowing work to continue. Although he ultimately lost money on the contract, Cox kept going, employing new technology and methods as conditions dictated. By 1939, Cox and Metal Industries Ltd. (the company that he had sold out to in 1932) had successfully raised 45 of the 52 scuttled ships. The last, the massive , was raised from a record depth of 45 metres just before work was suspended with the start of the Second World War, before being towed to Rosyth where it was broken up in 1946. A Morse key recovered from the battleship "Grosser Kurfürst" during the salvage is displayed at a Fife museum. Primarily because of its great distance from German airfields, Scapa Flow was again selected as the main British naval base during the Second World War. The strong defences built during the First World War had fallen into disrepair. Defence against air attack was inadequate and blockships 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, penetrated Scapa Flow and sank the First World War-era battleship 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 hole in the "Royal Oak", which flooded and quickly capsized. Of the 1,400-man crew, 833 were lost.