A tunnel is a long, narrow, essentially linear excavated underground opening, the length of which greatly exceeds its width or height. (Walhstrom, 1973). In contrast, a cavern is an underground opening whoselength and width are roughly similar. These two types of excavations are end-members, and every underground excavation undertaken by mankind can be looked upon as a combination of the two. A shaft can simply be thought of as a tunnel which is vertical rather than horizontal.