Plans for a New U-Boat Bunker at the Howaldtswerke yards on the River Elbe at Hamburg were approved in 1940, with construction commencing at the end of that year. Erected by the firm of Dyckerhoff & Widmann AG at the east end of the Vulkan basin, the bunker was completed in March 1941 and consisted of two pens. Each pen was 112m in length and 22.5m in width and could accommodate three boats moored side by side. To the rear of the bunker at ground level were a storage area on the west side, and offices and electrical switch-gear on the eastern side. The entire upper floor of this rear area was fitted out with workbenches and various equipment such as drills, vertical and horizontal boring machines, and lathes. Landward access was via small, well-protected steel doors on each side of the bunker. The Elbe II bunker was used primarily to provide cover for new Type XXI U-Boats being fitted out, and for existing boats returning to the yard for refit or repair. Despite numerous Allied bombing raids, the boats in the interior of the bunker remained safe, though some damage to the roof of the structure was achieved. On the night of 8th March 1945 a force of over 300 RAF bombers attacked the Hamburg dock area, dropping almost 1,000 tons of bombs. This was only the start of a concentrated bombing campaign against the dock installations in Hamburg. At the end of March, the RAF returned with a force of over 450 bombers and dropped over 2,200 tons of bombs on the area, laying waste to the Howaldtswerke yards but doing little or no damage to the bunker. The massive hanging steel doors covering the entrance were blown off during an air raid on 8th April 1 945, when once again over 400 bombers attacked the area. Six submarines, U-2505, U-3004, U-2501 and U-3506 (all Type XXIs) plus U-684 and U-685 (Type VIIC’s), were sheltering in the Elbe II hunker when the war ended. The Type XXIs were all awaiting repair and the Type VIICs were not fully completed. All six boats were crammed into the westernmost pen. Following the German agreement surrender the city of Hamburg to British forces on 3rd May the crews of the type XXI's made immediate preparations to scuttle their boats using explosive U-3506, U-3004 and U-250S were scuttled inside the pen, the others were moved out into the harbour waters before being sunk. The Elbe II bunker in Hamburg shortly before the end of the war with curtain, rather than portcullis, type of armoured doors sealing off the two boat entrances. The buildings on the roof were put there to overcome an acute shortage of space. The building towards the left, attached to the side of the blinker was a staircase. added to the outside to enable staff to get easily into the buildings on the roof. This was not bombproof and has since disappeared. The buildings along the bottom were also demolished, but quickly replaced by hilts to provide additional office and administration space for the shipyard. The interior of Elbe II, looking out into the Vulkanhafen at low tide to reveal the presence of U2501, a fourth type XXI, scuttled in front of the bunker. The majority of armoured doors can be seen stacked neatly towards the left of the opening, but one section has come adrift from the others to hang at a precarious angle from the ceiling.