cave

Limestone formations in the Lehman Caves, Nevada, USA. The caves – which are really a single cave extending a quarter-mile along the base of the Snake Range mountains – were discovered in about 1885 by Absalom Lehman, a rancher and miner. As well as familiar stalactite and stalagmite formations, the Lehman Caves feature some rarities such as the shields with graceful stalactite draperies pictured here.

Passage or tunnel – or series of tunnels – formed underground by water or by waves on a coast. Caves of the former type commonly occur in areas underlain by limestone, such as Kentucky, USA, and many Balkan regions, where the rocks are soluble in water. A pothole is a vertical hole in rock caused by water descending a crack; it is thus open to the sky.

Coastal caves are formed where rocks with lines of weakness, like basalt at tide level, are exposed to severe wave action. The erosion (corrasion and corrosion) of the rock layers is increased by subsidence, and the hollow in the cliff face grows still larger because of air compression in the chamber (hydraulic action). Where the roof of a cave has fallen in, the vent up to the land surface is called a blowhole. If this grows, finally destroying the cave form, the outside pillars of the cave are known as stacks or columns. The Old Man of Hoy (137 m/449 ft high), in the Orkney Islands, is a fine example of a stack.

Most inland caves are found in karst (limestone) regions, because limestone is soluble when exposed to acid water. As the water makes its way along the main joints, fissures, and bedding planes, they are constantly enlarged into potential cave passages, which ultimately join to form a complex network. Stalactites and stalagmites form due to water that is rich in calcium carbonate dripping from the roof of the cave. The collapse of the roof of a cave produces features such as natural arches and steep-sided gorges.

Limestone caves are usually found just below the water-table, wherever limestone outcrops on the surface. The biggest cave in the world is over 70 km/43 mi long, at Holloch, Switzerland.

Cave animals often show loss of pigmentation or sight, and under isolation, specialized species may develop. The scientific study of caves is called speleology.

Celebrated coastal caves include Fingal's Cave, Scotland, which has formed in a range of basalt columns.