In early April 1943, an Allied agent reported "enormous trenches" being excavated at the Watten site, and on 16 May 1943 an RAF reconnaissance mission led to Allied photographic interpreters noticing unidentified activity there.
Other large facilities were observed to be under construction elsewhere in the Pas-de-Calais.
The purpose of the construction works was very unclear; Lord Cherwell, Winston Churchill's scientific adviser, admitted that he had little idea what "these very large structures similar to gun emplacements" were but he believed that "if it is worth the enemy's while to go to all the trouble of building them it would seem worth ours to destroy them."

At the end of May, the British Chiefs of Staff ordered that aerial attacks be carried out against the so-called "heavy sites" being built by the Germans.
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.
The British Chiefs of Staff noted that a daylight attack by US bombers was under consideration but they raised objections to the proposal, as the Air Staff thought that Watten had nothing to do with rockets, suggesting that instead it might be merely a "protected operations room".

The timing of the first raid was influenced by advice given by Sir Malcolm McAlpine, the chairman of the construction company Sir Robert McAlpine, who suggested that the Watten site should be attacked while the concrete was still setting.
On 27 August 1943, 187 B-17 Flying Fortresses of the US 8th Air Force attacked the site with devastating effect.
The fortified train station on the north side of the bunker was especially badly damaged, as concrete had just been poured there.
Dornberger later wrote that following the attack the site was "a desolate heap of concrete, steel, props and planking.
The concrete hardened.
After a few days the shelter was beyond saving.
All we could do was roof in a part and use it for other work."
The bombing killed and injured hundreds of the slave workers on site; although the Allies had sought to avoid casualties by timing the raid with what they thought was a change of shifts, the shift pattern had been changed by the Germans at the last minute to achieve the day's work quota.

Only 35% of the Watten bunker had been completed by this time.
It was clearly no longer possible to use it as a launch site, but the Germans still needed LOX production facilities to supply V-2 sites elsewhere.
After surveying the site in September and October 1943, Organisation Todt engineers determined that the northern part of the facility was irretrievably damaged but decided to focus on completing the southern part to serve as a LOX factory.

One of OT's engineers, Werner Flos, came up with an idea to protect the bunker from bombardment by building it up from the roof first.
This was done by initially constructing a concrete plate, flat on the ground, which was 5 metres (16 ft) thick and weighed 37,000 tons.
It was incrementally raised by hydraulic jacks and then supported by walls which were built underneath it as it was raised, becoming the roof.
The resulting concrete cavern was intended to be used by the Germans as a bombproof liquid oxygen factory.
The thickness of the roof was chosen on the assumption that Allied bombs were incapable of penetrating such a depth of concrete; the Germans, however, were unaware of the British development of earthquake bombs.

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.
This project was expanded to turn the quarry into a fixed launch 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.
The Allies carried out further hea
In early April 1943, an Allied agent reported "enormous trenches" being excavated at the Watten site, and on 16 May 1943 an RAF reconnaissance mission led to Allied photographic interpreters noticing unidentified activity there. Other large facilities were observed to be under construction elsewhere in the Pas-de-Calais. The purpose of the construction works was very unclear; Lord Cherwell, Winston Churchill's scientific adviser, admitted that he had little idea what "these very large structures similar to gun emplacements" were but he believed that "if it is worth the enemy's while to go to all the trouble of building them it would seem worth ours to destroy them."
At the end of May, the British Chiefs of Staff ordered that aerial attacks be carried out against the so-called "heavy sites" being built by the Germans. 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. The British Chiefs of Staff noted that a daylight attack by US bombers was under consideration but they raised objections to the proposal, as the Air Staff thought that Watten had nothing to do with rockets, suggesting that instead it might be merely a "protected operations room".
The timing of the first raid was influenced by advice given by Sir Malcolm McAlpine, the chairman of the construction company Sir Robert McAlpine, who suggested that the Watten site should be attacked while the concrete was still setting. On 27 August 1943, 187 B-17 Flying Fortresses of the US 8th Air Force attacked the site with devastating effect. The fortified train station on the north side of the bunker was especially badly damaged, as concrete had just been poured there. Dornberger later wrote that following the attack the site was "a desolate heap of concrete, steel, props and planking. The concrete hardened. After a few days the shelter was beyond saving. All we could do was roof in a part and use it for other work." The bombing killed and injured hundreds of the slave workers on site; although the Allies had sought to avoid casualties by timing the raid with what they thought was a change of shifts, the shift pattern had been changed by the Germans at the last minute to achieve the day's work quota.
Only 35% of the Watten bunker had been completed by this time. It was clearly no longer possible to use it as a launch site, but the Germans still needed LOX production facilities to supply V-2 sites elsewhere. After surveying the site in September and October 1943, Organisation Todt engineers determined that the northern part of the facility was irretrievably damaged but decided to focus on completing the southern part to serve as a LOX factory.
One of OT's engineers, Werner Flos, came up with an idea to protect the bunker from bombardment by building it up from the roof first. This was done by initially constructing a concrete plate, flat on the ground, which was 5 metres (16 ft) thick and weighed 37,000 tons. It was incrementally raised by hydraulic jacks and then supported by walls which were built underneath it as it was raised, becoming the roof. The resulting concrete cavern was intended to be used by the Germans as a bombproof liquid oxygen factory. The thickness of the roof was chosen on the assumption that Allied bombs were incapable of penetrating such a depth of concrete; the Germans, however, were unaware of the British development of earthquake bombs.
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. This project was expanded to turn the quarry into a fixed launch 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. The Allies carried out further hea
Blockhaus d'Éperlecques