Abstract: In a process of lowering building structures, such as shutdown nuclear power plants or building structures which are to be erected as they are lowered, a caisson is constructed below the bed plate of the building structure. The caisson has a reinforced top plate for supporting the building structure by means of supporting elements and a reinforced concrete ring which depends from the rim of the top plate to constitute a cutting edge. A transfer duct, which is lowered with the top plate, is provided adjacent the rim of the top plate. Superatmospheric pressure is maintained by means of air locks in a working chamber which is defined under the top plate and enclosed by the ring.