in May 1944 alone. In response to Hitler's desire to see the site completed the workforce was expanded substantially from 1,100 in April 1944 to nearly 1,400 by June. About 60% of the workers were Germans; skilled workers, such as miners from Westphalia, were recruited to excavate the tunnels and build the dome. The remainder were principally Frenchmen conscripted by the Service du travail obligatoire (STO), plus Soviet prisoners of war. The project was overseen by several large German construction companies, with Philipp Holzman A.G. of Frankfurt am Main and the Grossdeutsche Schachtbau and Tiefbohr GmbH serving as the chief contractors. One of the most difficult challenges faced by the Germans was constructing the great dome while under regular air attack. The dome's designer, Todt Organisation engineer Werner Flos, devised a plan under which the dome would be built first, flat upon the ground, and the soil underneath it would be excavated so that the construction works below would be protected against aerial attacks. A circular trench was excavated on the top of the hill above the quarry to an outside diameter of . The dome was built within this trench and the galleries and octagonal preparation chamber were excavated below. As an additional bomb-proofing method, the dome was surrounded by a bomb-proof "skirt" or Zerschellerplatte of steel-reinforced concrete, wide and thick. This was supported by a series of buttresses, which were not tied into the dome itself, above the entrances to the Gustav and Gretchen tunnels. Another concrete structure was tied into the skirt to the north-west of the dome, which was perhaps intended for use as an observation and control tower. A separate underground building was constructed on the western side of the quarry to serve as a hospital and as offices for the engineers. A Decauville narrow-gauge railway was installed on the quarry floor to transport supplies from the main line to the construction site. A cube-shaped concrete building was constructed on the top of the hill, next to the dome. This was intended to be used as the bomb-proof outlet for a ventilation and air conditioning shaft. It was an essential component of a facility where dangerous and explosive gases were expected to be used in large quantities on a daily basis. It was never finished, and the Allies found when they captured the site that the ventilation shaft had not been fully excavated. The building