After the war, the Mimoyecques site lay abandoned.
Much of the equipment left by the Germans was disposed of as scrap metal.
A complete set of four steel plates, weighing 60 tons, that were intended to protect the entrances to the drifts were bought by the manager of the Hidrequent-Rixent quarries to be cut up for use in rock-crushing machinery.
Rediscovered by local historians in the 1990s, they remained at the quarries until 2010, when the surviving plates were returned to Mimoyecques, where they are now on display.

Despite the closure of the railway tunnel entrances it was still possible for many years to get into the complex by climbing down one of the inclined drifts.
In 1969, Marie-Madeleine Vasseur, a farmer from Landrethun, had the southern entrance excavated so that the tunnels could be used as a mushroom farm.
30 metres (98 ft) of the southern tunnel had to be removed to clear the blockage; the entrance now visible is not the original one built by the Germans.
The southern entrance had been bricked up again by the 1970s.
Moved to discover this forgotten construction, Vasseur, helped by family and friends, cleared the tunnels and installed an electricity supply.
The société à responsabilité limitée "La Forteresse de Mimoyecques" was constituted in 1984 to operate the site as a museum under the name of Forteresse de Mimoyecques – Un Mémorial International.

The museum closed at the end of the 2008 season when the owner retired.
Subsequently, the nonprofit organisation Conservatoire d'espaces naturels du Nord et Pas-de-Calais (Conservatory of natural sites of the Nord and Pas-de-Calais) purchased it at a cost of €330,000, with funding provided by the Nord-Pas-de-Calais regional council, the European Union and a private benefactor.
The Conservatory's interest was due to the presence on the site of a large bat colony that included rare species, such as the Greater Horseshoe Bat, Geoffroy's Bat and the Pond Bat.

The intercommunality of the Terre des Deux Caps and the authorities in nearly Landrethun set up a partnership to operate the site under the management of the existing museum of La Coupole near Saint-Omer.
The director of the latter, historian Yves le Maner, designed the contents of a new museum that was constructed at a cost of €360,000.

The site reopened to the public on 1 July 2010.
As well as presenting a history of the V-weapons and of the site, the museum enables visitors to see some of the tunnels and a mock-up of the HDP gun.
The tunnels also house memorials to Joseph Kennedy, the other bomber crew members killed during raids on the site, and the forced labourers who lost their lives during construction.
In 2011, the museum had about 11,000 visitors, of whom 53% were French, 18% Belgian and 16% British.
After the war, the Mimoyecques site lay abandoned. Much of the equipment left by the Germans was disposed of as scrap metal. A complete set of four steel plates, weighing 60 tons, that were intended to protect the entrances to the drifts were bought by the manager of the Hidrequent-Rixent quarries to be cut up for use in rock-crushing machinery. Rediscovered by local historians in the 1990s, they remained at the quarries until 2010, when the surviving plates were returned to Mimoyecques, where they are now on display.
Despite the closure of the railway tunnel entrances it was still possible for many years to get into the complex by climbing down one of the inclined drifts. In 1969, Marie-Madeleine Vasseur, a farmer from Landrethun, had the southern entrance excavated so that the tunnels could be used as a mushroom farm. 30 metres (98 ft) of the southern tunnel had to be removed to clear the blockage; the entrance now visible is not the original one built by the Germans. The southern entrance had been bricked up again by the 1970s. Moved to discover this forgotten construction, Vasseur, helped by family and friends, cleared the tunnels and installed an electricity supply. The société à responsabilité limitée "La Forteresse de Mimoyecques" was constituted in 1984 to operate the site as a museum under the name of Forteresse de Mimoyecques – Un Mémorial International.
The museum closed at the end of the 2008 season when the owner retired. Subsequently, the nonprofit organisation Conservatoire d'espaces naturels du Nord et Pas-de-Calais (Conservatory of natural sites of the Nord and Pas-de-Calais) purchased it at a cost of €330,000, with funding provided by the Nord-Pas-de-Calais regional council, the European Union and a private benefactor. The Conservatory's interest was due to the presence on the site of a large bat colony that included rare species, such as the Greater Horseshoe Bat, Geoffroy's Bat and the Pond Bat.
The intercommunality of the Terre des Deux Caps and the authorities in nearly Landrethun set up a partnership to operate the site under the management of the existing museum of La Coupole near Saint-Omer. The director of the latter, historian Yves le Maner, designed the contents of a new museum that was constructed at a cost of €360,000.
The site reopened to the public on 1 July 2010. As well as presenting a history of the V-weapons and of the site, the museum enables visitors to see some of the tunnels and a mock-up of the HDP gun. The tunnels also house memorials to Joseph Kennedy, the other bomber crew members killed during raids on the site, and the forced labourers who lost their lives during construction. In 2011, the museum had about 11,000 visitors, of whom 53% were French, 18% Belgian and 16% British.
Fortress of Mimoyecques