The Allies first noticed construction activity at Wizernes in mid-August 1943 when the Germans began building railway track and the offloading stores into the old quarry. After Hitler authorised the decision to turn the depot into a missile launch site, construction was stepped up. Work on the dome began in November 1943 and tunnelling in the cliff face below began in December. At the start of January, Allied reconnaissance aircraft observed an elaborate system of camouflage on the hill top, installed to conceal the dome. The building works were greatly hindered by the constant air-raid warnings, which stopped work 229 times 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.