Carlsbad Cavern was eaten out from below by sulfuric acid
Most limestone caves are dissolved by rainwater trickling down from the surface. New Mexico's Carlsbad Cavern and Lechuguilla Cave seem to have formed the other way round. Hydrogen sulfide rising from oil reservoirs mixed with groundwater, made sulfuric acid, and ate the rock away from underneath.
Caves are natural hollows beneath the surface, and they have been spotted on other rocky worlds such as Mars. Their formation, called speleogenesis, can take millions of years and may involve chemistry, flowing water, tectonic movement, pressure and even microbes. Geologists can date cave sediments with isotopes to reconstruct that history. Rock pressure probably prevents any cave from lying more than about 3,000 metres below the surface directly above it, although a cave's total depth from top entrance to lowest point can exceed that where the land above is high.
Solutional, or karst, caves are the most common kind. Most are in limestone, though chalk, dolomite, marble, gypsum and salt can host them too. Groundwater carrying carbonic acid and natural organic acids seeps along joints and faults, widening cracks into passages over time and producing a karst landscape of sinkholes and underground drainage. Slowly deposited calcium carbonate then decorates the chambers with stalactites, stalagmites, flowstone, soda straws and helictites, known collectively as speleothems. Anything below the water table stays flooded.
Some caves are as old as the rock around them. The commonest of these primary caves are lava tubes: a flow's surface cools into a crust that insulates the molten rock beneath, and when the lava drains out, a hollow tunnel remains. Examples occur in the Canary Islands, on Jeju-do and in eastern Idaho, and Kazumura Cave near Hilo in Hawaii runs 65.6 kilometres.
Along coasts, waves exploit weak spots in cliffs such as faults to carve sea caves, usually 5 to 50 metres long but occasionally beyond 300. Some now sit above the waterline after uplift, while in Thailand's Phang Nga Bay the sea has flooded older solution caves. Caves with streams flowing through are called active, and those left dry are relict. Studying them is speleology; exploring them for fun goes by caving, potholing or spelunking.
Source: Cave
