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The outside world viewed from a cave

A cave is a natural underground void large enough for an adult human to enter. Some scientists stipulate that it must be large enough that some portion of it will not receive daylight; however, in popular usage, the term includes smaller spaces like cliff cavities, rock shelters and sea caves.


Cave types and formation

Caves are formed by geologic processes. These may involve a combination of chemical processes, tectonic forces and atmospheric influences.

Primary caves

Some caves are formed at the same time as the surrounding rock. These are called primary caves.

Lava tube cave at
Lava tube cave at Craters of the Moon
  • Lava tubes are formed through volcanic activity. They are the most common primary caves. Lava flows downhill and the surface cools down and becomes hard. The lava now flows inside its crust, until the eruption ends.

The liquid lava inside the crust flows out and leaves a hollow tube. The most important lava tubes are found on Hawaii (Big Island). Kazumura Cave near Hilo is the longest and deepest lava tube of the world and also the eighth longest cave of the United States.

  • Blister caves are also formed through volcanic activity.

Secondary caves

Secondary caves are formed inside the rock after the rock itself has formed by processes which removes material such as solution and erosion.

  • Sea caves are very common at all coasts of the world, but as they are restricted to the zone where waves work on the rocks of the coast they are generally rather small.
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Ice cave in Big Four Glacier, Big Four Mountain, Washington, ca. 1920
  • Ice caves occur in and under glaciers, formed by melting. They are also influenced by the very slow flow of the ice which tends to close the caves again.
  • Solutional caves may form anywhere with rock which is soluble, and are most prevalent in limestone, but can also form in other material, including chalk, dolomite, marble, loess, ice, granite, salt, lava, sandstone, and gypsum. The most common process of cave formation is karstification, which is the solution of rocks by rain water.
    • 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.
  • Caverns are large caves (or large chambers in caves).

Cave formation in limestone occurs because limestone dissolves under the action of rainwater and groundwater charged with CO2 (carbonic acid) and naturally occurring organic acids. The dissolution process produces a distinctive landform known as karst and characterized by sinkholes, sinking streams, and underground drainage.

Limestone solution is an important process in cave formation and the origin of the great majority of all caves on Earth. The reasons for this abundance are the facts that limestone is so common and the slowness of the solution process. If it were faster, the lifespan of limestone caves would be much shorter and their number much lower.

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Speleothems in Hall of the Mountain Kings, Ogof Craig a Ffynnon, South Wales

Limestone caves are often adorned with calcium carbonate formations produced through slow precipitation, including the most common and well-known stalactites and stalagmites. These secondary mineral deposits in caves are called speleothems. The world's most spectacularly decorated cave is generally regarded to be Lechuguilla Cave (New Mexico, USA).

Lechuguilla and nearby Carlsbad Caverns are now believed to be examples of another type of solutional cave. They were formed by acid rising from below, where reservoirs of oil give off sulfurous fumes, rather than by acidic water percolating from the surface.


Caves are sparse in South America, Africa, and Antarctica, but are found widely in Europe, Asia, and North America.

The distribution of cave systems so far discovered is widely skewed toward countries where caving is popular (such as the United States, France, Italy, the UK etc.). It is likely that many more systems remain to be discovered, especially in China, which, despite containing around half the world's exposed limestone - more than 1,000,000 km2 - has hardly been explored underground.

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Fracture blocks in Seneca Caverns, Ohio, USA


Cave inhabiting animals can be categorized as troglobites (cave limited-species), troglophiles (species which can live their entire lives in caves, but also occur in other environments), trogloxenes (species which utilize caves, but must leave the caves to complete their life cycle) and accidentals. Some authors use separate terminology for aquatic forms (i.e., stygobites, stygophiles, stygoxenes).

Of these animals, the troglobites are among the most extraordinary of organisms. Troglobitic species often show a suite of characters, termed troglomorphies, associated with their adaptation to subterranean life. Among these characters are a loss of pigment (often resulting in a pale or white coloration), loss of eyes (or at least of optical functionality), elongation of appendages, and an enhancement of other senses (such as ability to sense vibrations in water). Aquatic troglobites (or stygobites), such as the endangered Alabama cave shrimp, live in bodies of water found in the caves and are fed by detritus washed into the caves, and by the feces of bats and other cave inhabitants.

Bats, like the Gray bat and Mexican Free-tailed Bat, are trogloxenes, and are commonly found in caves, but forage outside of the caves. Some species of cave crickets are classified as trogloxenes, as they roost in caves by day and forage above ground at night.

Because of the fragile nature of the cave ecosytem, and the fact that cave regions tend to be isolated from one another, caves harbor a number of endangered species, such as, for example the Tooth cave spider and the Gray bat.

Caves are visited by many surface-loving animals, including humans. These are usually relatively short-lived incursions, due to the lack of light and sustenance.


Since new caves are continually being explored, the various records of cave dimensions need to be updated fairly frequently.

The system with the greatest (by some distance) total length of passage is Mammoth Cave (Kentucky, USA) at 579km in length. This record is unlikely to be surpassed in the near future as the next most extensive known cave is the Optimisticeskaja system in the Ukraine, at 214km.

As of 2005, the deepest known cave (measured from its highest entrance to its lowest point) is Voronya Cave (Abkhazia, Georgia), with a depth of 2,080m. This was the first cave to be explored to a depth of more than 2km. (The first cave to be descended below 1km was the famous Gouffre Berger in France). The Gouffre Mirolda - Lucien Bouclier cave in France (1733m) and the Lamprechtsofen Vogelschacht Weg Schacht in Austria (1632m) are the current second and third deepest caves. This particular record has changed several times in recent years.

The deepest individual pitch (vertical drop) within a cave is 603m in the Vrtoglavica cave in Slovenia, followed by Patkov Gušt (553m) in the Velebit mountain, Croatia.

The largest individual cavern ever discovered is the Sarawak Chamber, in the Gunung Mulu National Park ( (Sarawak, Borneo, Malaysia), a sloping, boulder strewn chamber with an area of approximately 600m by 400m and a height of 80m.

For a list of the world's notable caves, see list of caves.

Archaeological and social importance

Throughout history, primitive peoples have made use of caves for shelter, burial, or as religious sites. Since items placed in caves are protected from the climate and scavanging animals, this means caves are an archaeological treasure house for learning about these people. Cave paintings are of particular interest. One example is the Great Cave of Niah, which contains evidence of human habitation dating back 40,000 years.

Caves are also important for geological research because they can reveal details of past climactic conditions in speleothems and sediment layers.

Caves are frequently used today as sites for recreation. Caving, for example, is the popular sport of cave exploration. For the less adventurous, a number of the world's prettier and more accessible caves have been converted into show caves, where artificial lighting, floors, and other aids allow the casual visitor to experience the cave with minimal inconvenience. Caves have also been used for BASE jumping and cave diving.

See also


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