The bunker, built by Nazi Germany under the codename Kraftwerk Nord West (Powerplant Northwest)[6] between March 1943 and July 1944, was originally intended to be a launching facility for the V-2 (A-4) ballistic missile.

The bunker was never completed as a result of the repeated bombing by the British and United States air forces as part of Operation Crossbow against the German V-weapons programme.

[4] Various concepts were mooted for the A-4's deployment in a March 1942 study by Walter Dornberger, the head of the A-4 development project at the Peenemünde Army Research Center.

He suggested that the missiles should be based in heavily defended fixed sites of a bunker-style design similar to the massive submarine pens then under construction in occupied France and Norway.

A high rate of fire could be sustained as the facility could effectively operate like a production line, sending a steady flow of missiles to the launch pads.

[10] The German Army preferred an alternative approach which would use trailer-style mobile launch platforms called Meillerwagen accompanied by testing and fuelling equipment mounted on railway cars or trucks.

Although this configuration was far less efficient and would have a much lower rate of fire, it would have the great advantage of presenting a much smaller target for the Allied air forces.

The Army was not convinced that fixed bunkers could resist repeated air attacks and was particularly concerned about the vulnerability of the launch sites' road and rail links, which were essential for resupplying them with missiles and fuel.

[4] In November 1942, Hitler and Minister of Munitions Albert Speer discussed possible launch configurations and examined models and plans of the proposed bunkers and mobile launchers.

[4] In December 1942, Speer ordered Peenemünde officers and engineers (including Colonel Gerhard Stegmair,[12] Dr Ernst Steinhoff and Lieutenant-Colonel Georg Thom) to tour the Artois region in northwest France and locate a suitable site for an A-4 launch facility.

[16] Situated 177 km (110 mi) from London, it was far enough inland to be safe from naval guns and it was sheltered to an extent by a ridge that rises to a height of 90 m (300 ft) to the north.

[3] When US Army Major General Lewis H. Brereton inspected the site after it had been captured by the Allies, he described the bunker as "more extensive than any concrete constructions we have in the United States, with the possible exception of Boulder Dam.

[16] The workforce consisted of a mixture of German specialists and forcibly conscripted Frenchmen from the Service du Travail Obligatoire (STO).

They were supplemented by Belgian, Dutch, French, Polish, Czech and Soviet prisoners of war and civilian conscripts, who were used as slave labour.

The living and working conditions were extremely harsh, especially for the political prisoners and the eastern Europeans, who were given especially punitive treatment due to their status as the most expendable members of the workforce.

[28] For the non-German workers, falling ill or being unable to work through injury was the equivalent of a death sentence, as they would either be left to die or be transported back to the concentration camps from which they had been brought.

[21] Building materials were brought there by barges and trains where they were unloaded onto a Decauville narrow-gauge railway for transportation to the construction site, where concrete mixers operated day and night.

[2] On 6 August, Duncan Sandys, who headed a high-level Cabinet committee to coordinate the British defence against the German V-weapons, recommended that the Watten site should also be attacked because of the progress being made in its construction.

[33] The Germans' main focus of attention switched instead to Schotterwerk Nordwest, the former quarry at nearby Wizernes, where work had been ongoing to build a bombproof V-2 storage facility.

Plans were put into effect to build a huge concrete dome – now open to the public as the museum of La Coupole – under which missiles would be fuelled and armed in a network of tunnels before being transported outside for launching.

[20] The Allies carried out further heavy bombing against both the Watten and Wizernes sites with little initial effect on the buildings themselves, although the rail and road network around them was systematically destroyed.

[2] Three days later an Allied raid succeeded in wrecking the interior of the Watten bunker with a Tallboy bomb that brought down part of the roof.

The site itself was now useless, as the Germans recognised when they wryly codenamed it Concrete Lump, and the liquid oxygen generators and machinery were transferred to the Mittelwerk V-2 factory in central Germany, well away from Allied bombers.

[53] Based on the discovery of large aluminium tanks installed in the main part of the bunker, he opined that the Germans had intended to use it as a factory for the production of hydrogen peroxide for use in the fuelling of V-1 and V-2 missiles.

"[54] He recommended that (unlike the Mimoyecques and Wizernes sites) the Watten bunker presented no threat to the UK's security and "there is thus no imperative need, on that account, to ensure the destruction of the workings.

[56] The site had been chosen for testing purposes in October 1944 as it had the largest accessible interior area of the targets under consideration and was furthest from an inhabited town.

The British Assistant Military Attaché, Major W.C. Morgan, reported to the Director of Military Intelligence at the War Office that the main part of the bunker had not been significantly damaged by bombing and that although it was flooded, if it was patched and drained "the building could be quickly made ready to receive oxygen liquifying plant machinery, or for any other purpose requiring a large and practically bomb-proof building.

[7] The area around the bunker has been re-forested, though it is still heavily scarred by bomb craters, and various items of Second World War military equipment (including a V-1 on a launch ramp) are on display alongside paths around the site.

An open-air trail leads to and around the bunker with interpretative signs posted at various points to tell the story of the site and the German V-weapons programme.