The bunker was inspected on 10 September 1944 by the French atomic scientist Frédéric Joliot-Curie, accompanied by Sandys.
Following the visit, Sandys ordered a Technical Inter-Services Mission under Colonel T.R.B. Sanders to investigate the sites at Mimoyecques, Siracourt, Watten, and Wizernes, collectively known to the Allies as the "Heavy Crossbow" sites.
Sanders' report was submitted to the War Cabinet on 19 March 1945.

Despite the capture of Watten, it was still not known at this time what the site had been intended for.
Sanders noted that "the purpose of the structures was never known throughout the period of intensive reconnaissance and attack".
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.
He ruled out the possibility that it could have been used for LOX production and concluded, erroneously, that "the site had no offensive role."
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."

The bunker was targeted again by the Allies in February 1945, this time to test the newly developed CP/RA Disney bomb – a 4,500 lb (2,000 kg) concrete-piercing rocket-assisted bomb designed to double the normal impact velocity, and thereby increase the penetration, of the projectile.
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.

On 3 February 1945, a B-17 of the US Eighth Air Force dropped a Disney bomb on the Watten bunker and scored a hit over the wall section, but the results were inconclusive and the Air Force was not able to determine how well the bomb had penetrated the concrete.
Although Disney bombs were used operationally on a number of occasions, the weapon's introduction came too late to be of any significance in the war effort.
In January 2009 the body of the Disney bomb was extracted from the roof, where it had embedded itself.

The Watten bunker was inspected again on 20 June 1951 by an Anglo-French commission to determine whether it was capable of being reused for military purposes.
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."

No further military use was made of the bunker and the land on which it stands reverted to private ownership.
It was left abandoned for many years before the owners decided to redevelop the site.
In 1973, the bunker was opened to the public for the first time under the name of Le Blockhaus d'Éperlecques.
The ownership was taken over by Hubert de Mégille in the mid-1980s and on 3 September 1986 the French state declared it a monument historique.
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.
In 2009, the museum welcomed 45,000 visitors.
The bunker was inspected on 10 September 1944 by the French atomic scientist Frédéric Joliot-Curie, accompanied by Sandys. Following the visit, Sandys ordered a Technical Inter-Services Mission under Colonel T.R.B. Sanders to investigate the sites at Mimoyecques, Siracourt, Watten, and Wizernes, collectively known to the Allies as the "Heavy Crossbow" sites. Sanders' report was submitted to the War Cabinet on 19 March 1945.
Despite the capture of Watten, it was still not known at this time what the site had been intended for. Sanders noted that "the purpose of the structures was never known throughout the period of intensive reconnaissance and attack". 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. He ruled out the possibility that it could have been used for LOX production and concluded, erroneously, that "the site had no offensive role." 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."
The bunker was targeted again by the Allies in February 1945, this time to test the newly developed CP/RA Disney bomb – a 4,500 lb (2,000 kg) concrete-piercing rocket-assisted bomb designed to double the normal impact velocity, and thereby increase the penetration, of the projectile. 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.
On 3 February 1945, a B-17 of the US Eighth Air Force dropped a Disney bomb on the Watten bunker and scored a hit over the wall section, but the results were inconclusive and the Air Force was not able to determine how well the bomb had penetrated the concrete. Although Disney bombs were used operationally on a number of occasions, the weapon's introduction came too late to be of any significance in the war effort. In January 2009 the body of the Disney bomb was extracted from the roof, where it had embedded itself.
The Watten bunker was inspected again on 20 June 1951 by an Anglo-French commission to determine whether it was capable of being reused for military purposes. 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."
No further military use was made of the bunker and the land on which it stands reverted to private ownership. It was left abandoned for many years before the owners decided to redevelop the site. In 1973, the bunker was opened to the public for the first time under the name of Le Blockhaus d'Éperlecques. The ownership was taken over by Hubert de Mégille in the mid-1980s and on 3 September 1986 the French state declared it a monument historique. 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. In 2009, the museum welcomed 45,000 visitors.
Blockhaus d'Éperlecques