A tunnel is an underground or underwater passageway, dug through the surrounding soil/earth/rock and enclosed except for entrance and exit, commonly at each end. Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and shape of the tunnel and appropriate risk management. There are three basic types of tunnel construction in common use: Cut-and-cover tunnel, constructed in a shallow trench and then covered over. Bored tunnel, constructed in situ, without removing the ground above. They are usually of circular or horseshoe cross-section. Immersed tube tunnel, sunk into a body of water and laid on or buried just under its bed.