These words refer to a hollow or opening in the earth, either natural or artificial. Cave is least specific and can be applied to any hollowed-out area in the earth, usually one that occurs in the side of a mountain, cliff or hill. The cliff dwellers found that natural caves gave them warmth and protection during the Ice Age. Bears, for similar reasons, hibernate in caves during the winter.

Cavern , when used in place of cave, tends to sound inflated in diction. It can be used accurately, however, to refer to a more extensive subterranean cave or set of caves indefinite in extent, especially when it suggests a natural chamber such as one formed in limestone by running water. In this case, the sense of an opening into the slanted face of a hill is not necessarily maintained. One or more mouths of the cavern may open on to relatively level ground: The tourists clutched the guide-rail and of light. Cavern may be also used metaphorically for any obscure recess: the dark caverns of his mind.

Grotto is the most specific of these terms, though it can refer either to an artificial or natural hollow. When artificial, it is a cavelike, manmade structure built as a recreational retreat or shrine: The towns-people placed a Christmas tree within their quaintly decorated grotto and sang carols there throughout the holidays. When natural, a grotto is typically picturesque, often forming a recess in a cavern , one that may be filled with odd-shaped stalagmites and stalactites: Daylight has never touched the cave paintings in the grotto at Lascaux.

Burrow and tunnel are alike in suggesting an artificial opening in the earth and often implying a linear shape as contrasted with the roughly spherical hollow most typical of a cave . Burrow refers to the hollow dug by some animal such as a rabbit: The burrows of rabbits can cause erosion of pasture land. Tunnel can refer to a hollow in the earth dug by an animal, but it is used more frequently to refer to one dug by man. If its purpose is to permit movement from one point to another, as in a subway system, a tunnel would be mainly horizontal over most of its course. If dug for purposes of mining, a tunnel would usually slant sharply downwards into the depths of the earth: The miners were trapped in the tunnel by a cave-in along the passage just behind them.