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A cave is a natural underground void large enough for a human to enter. Some people suggest that the term cave should only apply to cavities that have some part that does not receive daylight; however, in popular usage, the term includes smaller spaces like sea caves, rock shelters, and grottos.

Most caves are formed in limestone by dissolution.
Speleothems in Hall of the Mountain King, Ogof Craig a Ffynnon, South Wales.

* Solutional caves may form anywhere with rock that is soluble, and are most prevalent in limestone, but can also form in other material, including chalk, dolomite, marble, granite, salt, sandstone, fossilized coral and gypsum.
o The largest and most abundant solutional caves are located in limestone. Limestone dissolves under the action of rainwater and groundwater charged with H2CO3 (carbonic acid) and naturally occurring organic acids. The dissolution process produces a distinctive landform known as karst, characterized by sinkholes, sinking streams, and underground drainage. Limestone caves are often adorned with calcium carbonate formations produced through slow precipitation. This include: flowstones, stalactites, stalagmites, helictites, draperies, soda straws and columns. These secondary mineral deposits in caves are called speleothems.
o The world’s most spectacularly decorated cave is generally regarded to be Lechuguilla Cave in New Mexico. Lechuguilla and nearby Carlsbad Caverns are now believed to be examples of another type of solutional cave. They were formed by H2S (hydrogen sulfide) gas rising from below, where reservoirs of oil give off sulfurous fumes. This gas mixes with ground water and forms H2SO4 (sulfuric acid). The acid then dissolves the limestone from below, rather than from above, by acidic water percolating from the surface.

Exploring a lava tube in Hawaii

* Some caves are formed at the same time as the surrounding rock. These are sometimes called primary caves.
o Lava tubes are formed through volcanic activity and are the most common ‘primary’ caves. The lava flows downhill and the surface cools and solidifies. The hotter lava continues to flow under that crust, and if most of the liquid lava beneath the crust flows out, a hollow tube remains, thus forming a cavity. Examples of such caves can be found on Tenerife, Big Island, and many other places. Kazumura Cave near Hilo is a remarkably long and deep lava tube; it is 65.6 kilometers (40.8 mi) long.
o Blister caves are also formed through volcanic activity.

Painted Cave, a large sea cave, Santa Cruz Island, California

* Sea caves are found along coasts around the world. A special case is littoral caves, which are formed by wave action in zones of weakness in sea cliffs. Often these weaknesses are faults, but they may also be dykes or bedding-plane contacts. Some wave-cut caves are now above sea level because of later uplift. Elsewhere, in places such as Thailand’s Phang Nga Bay, solutional caves have been flooded by the sea and are now subject to littoral erosion. Sea caves are generally around 5 meters (16 ft) to 50 meters (160 ft) in length but may exceed 300 meters (980 ft).

Glacier cave in Big Four Glacier, Big Four Mountain, Washington, ca. 1920.

* Glacier caves occur in ice and under glaciers and are formed by melting. They are also influenced by the very slow flow of the ice, which tends to close the caves again. (These are sometimes called ice caves, though this term is properly reserved for caves that contain year-round ice formations).
* Fracture caves are formed when layers of more soluble minerals, such as gypsum, dissolve out from between layers of less soluble rock. These rocks fracture and collapse in blocks.
* Talus caves are the openings between rocks that have fallen down into a pile, often at the bases of cliffs.
* Anchihaline caves are caves, usually coastal, containing a mixture of freshwater and saline water (usually sea water). They occur in many parts of the world, and often contain highly specialized and endemic faunas.

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