1. Vertical, or steeply inclined, sections of a cave passage, of enormously varied size. The world's deepest known shaft is the entrance shaft of Brezno pod Velbom on the Kanin plateau, Slovenia; it is 501m deep, with no ledges. Much debate surrounds statistics on the depths of fully underground shafts, which may be broken by ledges, but among the deepest is a shaft about 430m deep in Italy's Abisso di Monte Novegno [9]. 2. A cylindrical tube generally steep sided, that forms by solution and (or) collapse [10]. 3. A vertical passage in a cave [10]. 4. A vertical and usually large diameter hole penetrating geologic formations for access of subsurface points [16]. See jama, karst shaft. See also pit; pothole (definition 2.).