The filter room of Immerhof. This work is an exception in the Maginot Line in the way that it was entirely constructed above the ground. All the galleries and the barracks were made of heavy concrete. Therefore this room has, very unusually, an anti-splitering roof. But this room has another special feature: it was built after the war. Originally the filters were installed at the lower level. However when the engines were replaced by larger and more powerful ones an engine was installed in the filter room. The filters were therefore placed on the upper floor.