In 1943 French agents reported that the Germans were planning to mount an offensive against the United Kingdom that would involve the use of secret weapons resembling giant mortars sunk in the ground and served by rail links.
The first signs of abnormal activity at Mimoyecques were spotted by analysts at the Allied Central Interpretation Unit in September 1943, when aerial reconnaissance revealed that the Germans were building railway loops leading to the tunnels into the eastern and western sites.
Further reconnaissance flights in October 1943 photographed large-scale activity around the tunnels.

An analyst named André Kenny discovered a series of shafts when he saw from a reconnaissance photograph that a haystack concealing one of them had disintegrated, perhaps through the effects of a gale, revealing the entrance, a windlass and pulley.
The purpose of the site was unclear, but it was thought to be some kind of shelter for launching rockets or flying bombs.
An MI6 agent reported that "a concrete chamber was to be built near one of the tunnels for the installation of a tube, 40 to 50 metres long, which he referred to as a 'rocket launching cannon'".
The shafts were interpreted as "air holes to allow for the expansion of the gases released by the explosion of the launching charge."

The Allies were unaware of the HDP gun and therefore of the Mimoyecques site's true purpose.
Allied intelligence believed at the time that the V-2 rocket had to be launched from tubes or "projectors", so it was assumed that the inclined shafts at Mimoyecques were intended to house such devices.

The lack of intelligence on Mimoyecques was frustrating for those involved in Crossbow operations to counter the V-weapons.
On 21 March 1944 the British Chiefs of Staff discussed the shortage of intelligence but were told by Reginald Victor Jones, one of the "Crossbow Committee" members, that little information was leaking out because the workforce was predominantly German.
The Committee's head, Duncan Sandys, pressed for greater efforts and proposed that the Special Operations Executive be tasked to kidnap a German technician who could be interrogated for information.
The suggestion was approved, but was never put into effect.

In the end the Chiefs of Staff instructed General Eisenhower to begin intensive attacks on the so-called "Heavy Crossbow" sites, including Mimoyecques, which was still believed to be intended for use as a rocket-launching site.

The Allied air forces carried out several bombing raids on Mimoyecques between November 1943 and June 1944 but caused little damage.
The bombing disrupted the construction project and the initial raids of 5 and 8 November 1943 caused work to be delayed for about a month.
The Germans subsequently decided to abandon the western site, where work had not progressed very far, and concentrated on the eastern site.
On 6 July 1944 the Royal Air Force began bombing the site with ground-penetrating Tallboy "earthquake" bombs.
One Tallboy hit the concrete slab on top of Drift IV, collapsing the drift.
Three others penetrated the tunnels below and substantially damaged the facility, causing several of the galleries to collapse in places.

Around 300 Germans and forced labourers were buried alive by the collapses.
Adding to the Germans' difficulties, major technical problems were discovered with the HDP gun projectiles.
They had been designed to exit the barrels at a speed of about 1,500 m (4,900 ft) per second, but the Germans found that a design fault caused the projectiles to begin "tumbling" in flight at speeds above 1,000 m (3,300 ft) per second, causing them to fall well short of the target.
This was not discovered until over 20,000 projectiles had already been manufactured.

After the devastating raid of 6 July, the Germans held a high-level meeting on the site's future at which Hitler ordered major changes to the site's development.
On 12 July 1944 he signed an order instructing that only five HDP guns were to be installed in a single drift.
The two others were to be reused to house a pair of Krupp K5 artillery
In 1943 French agents reported that the Germans were planning to mount an offensive against the United Kingdom that would involve the use of secret weapons resembling giant mortars sunk in the ground and served by rail links. The first signs of abnormal activity at Mimoyecques were spotted by analysts at the Allied Central Interpretation Unit in September 1943, when aerial reconnaissance revealed that the Germans were building railway loops leading to the tunnels into the eastern and western sites. Further reconnaissance flights in October 1943 photographed large-scale activity around the tunnels.
An analyst named André Kenny discovered a series of shafts when he saw from a reconnaissance photograph that a haystack concealing one of them had disintegrated, perhaps through the effects of a gale, revealing the entrance, a windlass and pulley. The purpose of the site was unclear, but it was thought to be some kind of shelter for launching rockets or flying bombs. An MI6 agent reported that "a concrete chamber was to be built near one of the tunnels for the installation of a tube, 40 to 50 metres long, which he referred to as a 'rocket launching cannon'". The shafts were interpreted as "air holes to allow for the expansion of the gases released by the explosion of the launching charge."
The Allies were unaware of the HDP gun and therefore of the Mimoyecques site's true purpose. Allied intelligence believed at the time that the V-2 rocket had to be launched from tubes or "projectors", so it was assumed that the inclined shafts at Mimoyecques were intended to house such devices.
The lack of intelligence on Mimoyecques was frustrating for those involved in Crossbow operations to counter the V-weapons. On 21 March 1944 the British Chiefs of Staff discussed the shortage of intelligence but were told by Reginald Victor Jones, one of the "Crossbow Committee" members, that little information was leaking out because the workforce was predominantly German. The Committee's head, Duncan Sandys, pressed for greater efforts and proposed that the Special Operations Executive be tasked to kidnap a German technician who could be interrogated for information. The suggestion was approved, but was never put into effect.
In the end the Chiefs of Staff instructed General Eisenhower to begin intensive attacks on the so-called "Heavy Crossbow" sites, including Mimoyecques, which was still believed to be intended for use as a rocket-launching site.
The Allied air forces carried out several bombing raids on Mimoyecques between November 1943 and June 1944 but caused little damage. The bombing disrupted the construction project and the initial raids of 5 and 8 November 1943 caused work to be delayed for about a month. The Germans subsequently decided to abandon the western site, where work had not progressed very far, and concentrated on the eastern site. On 6 July 1944 the Royal Air Force began bombing the site with ground-penetrating Tallboy "earthquake" bombs. One Tallboy hit the concrete slab on top of Drift IV, collapsing the drift. Three others penetrated the tunnels below and substantially damaged the facility, causing several of the galleries to collapse in places.
Around 300 Germans and forced labourers were buried alive by the collapses. Adding to the Germans' difficulties, major technical problems were discovered with the HDP gun projectiles. They had been designed to exit the barrels at a speed of about 1,500 m (4,900 ft) per second, but the Germans found that a design fault caused the projectiles to begin "tumbling" in flight at speeds above 1,000 m (3,300 ft) per second, causing them to fall well short of the target. This was not discovered until over 20,000 projectiles had already been manufactured.
After the devastating raid of 6 July, the Germans held a high-level meeting on the site's future at which Hitler ordered major changes to the site's development. On 12 July 1944 he signed an order instructing that only five HDP guns were to be installed in a single drift. The two others were to be reused to house a pair of Krupp K5 artillery 
Fortress of Mimoyecques