All the blocks of the works in the Maginot Line had their own ventilation system. Here the fans in block 3 of Pas du Roc. Since the work was built in a high mountain top the designers counted that a gas attack would have no effect, since all the gas would fall down to the valley. Therefore they gave less attention to the gas protection. In fact the filter room was not finished in 1940 with only the room excavated.