Germans decide to build a base of V2 in the North of France in Eperlecques. 
 
The works begin in March 1943, and by the way they move quickly. Numerous nationalities are present on this enormous construction site.    

A series of plans found in the German archives show the bunker of Eperlecques. An attached sketch of M.Y. Delefosse shows the current state of the Bunker with regard to the initial project.

It was essentially about a real assembly plant for V2 rockets. The latter was assembled in the part situated in the North, as we can see it on the plan (drawings of M.Y. Delefosse.) 
This part was served by two railroads, gathering Calais to Saint-Omer. 

Between these two ways was a garage for the road vehicles which reached the same bunker and could pass off there. This side of the building(ship), although very damaged by the later bombardments, is still visible today. More in the South was the hall of assembly and check of V2 rockets, and in the last part was planned the installation of five groups compressors of liquid oxygen manufacturing.

Once drawn up vertically in the gallery, the rocket was transferred and filled with its fuels and combustives. The rocket so equipped was ready to be sent. It crossed then the corridor and the revolving door 17.5 metres high, the place of which we still see. Then took place the firing.

It must be mentionned that there are chicanes on the side walls of this corridor. These chicanes are still visible, they were certainly intended to break the shock wave which had to penetrate into the corridor at the time of the departure of the rocket.

After the bombing of the Bunker of Eperlecques on 27th August 1943, German military authorities became aware of the vulnerability of their construction site. It had indeed become impossible to use the Bunker of Eperlecques as unique base to assemble and send rockets.

On the other hand, it was necessary to find immediately a solution to end the realization, at least the south part of the building which had practically escaped the allied bombs. 

The engineer Floss then had the extraordinary idea to pursue this construction by the method said "the tortoise": that is to make a concrete shell 5 metres in thickness, to pour it at the level where the constructions had managed at this moment, and then to lift  this shell by hydraulic jacks, what put the construction site shielded from bombardments, and allowed to build walls quietly.

Thanks to this technique the current Bunker was built. The roof, realized in several sectors, was lifted by successive stages, and we can even see today the concrete strata which were poured after every rise to form the outside wall. This part of the bunker had so risen 28 metre height, what created inside a volume being enough to install the factory of liquid oxygen, but also to serve as a shelter for the staff and as storage point for the rockets elements, for combustives and for fuels, necessary for their functioning.

  We can suppose that the German technicians had not completely given up using the bunker of Eperlecques, even in its reduced shape, as completely autonomous base for the assembly and the launch of some rockets, while being, according to the official recommendations, just a factory of liquid oxygen manufacturing.

The presence of several openings, at the top of the exit rockets corridor, to allow, doubtless, the evacuation of gases which risked to accumulate in this corridor, confirms this point of view.

It is towards the same period that,  in front of the impossibility to realize in Eperlecques a unique building allowing at the same time to go up rockets and to make some liquid oxygen, it was decided to transfer these assembly and launching studios under the dome of Wizernes, one ancient limestone quarry, so that factories or launching site were shielded from the allied bombardments.

A tallboy bomb fell directly on the North side of the building, the other one fell 27 metres near the South side.
On the North side we can still see the track of the impact inside, but the bomb did hardly not made damages for the building. It doubtless shook it. The crater made in the concrete was quickly repaired by the Germans, because at the time of the liberation by the allied troops, the coffering which had been used for this repair was still in place.