Abstract: The present invention relates generally to a method and a device for building in the ground vertical-walled tunnels or the like starting from a subterranean conduit, and also to the underground structures, such as tunnels or the like, obtained by the said method.Already known in the art is the making of underground vertical walls by first providing trenches lined with sheet metal and then filling the same with concrete or with stonework or brickwork. Such trenches are usually excavated by the open-cut method, starting from the surface of the ground or from an open excavation.Where such walls must be constructed on a surface-built urban site or at a location where communication facilities such as streets, roads, railway tracks, airport runways, etc., are established and the activities taking place at the surface must not be interfered with, the trenches may be started from underground horizontal propped galleries made by known conventional means.