As oil is found deep in the ground, its presence cannot be determined by a study of the surface. Consequently, a geological survey of the underground rock structure must be caried out. If it is thought that the rocks in a certain area contain oil, a "drilling rig" is assembled. The most obvious part of a drilling rig is called "a demick." It is used to lift sections of pipe, which are lowered into the hole made by the drill. As the hole is being drilled, a steel pipe is pushed down to prevent the sides from falling in. If oil is struck, a cover is firmly fixed to the top of the pipe and the oil is allowed to escape through a series of valves.