Cave

Caves and caverns are natural spaces under the surface of the Earth. Scientists have found similar features on other worlds like Mars. These amazing openings form mostly through the weathering of rock and can stretch far beneath our feet. Some smaller caves, called exogene caves, don't go very deep, like rock shelters. Others that go deeper than their opening are called endogene caves.

The study of caves and everything around them is called speleology. Many people enjoy visiting caves as a fun activity. This is sometimes called caving, potholing, or spelunking. Exploring these hidden spaces helps us learn about Earth's history and the unique life that can live inside caves.

Formation types

The formation and development of caves is known as speleogenesis. It can happen over millions of years. Caves come in many sizes and are formed by different geological processes. These may include chemical processes, water erosion, tectonic forces, microorganisms, pressure, and weather. Scientists can use special dating methods on cave sediments to learn about the history of how caves were formed.

Caves cannot be more than about 3,000 metres (9,800 ft) deep because of the weight of the rocks above. Most caves form in limestone through a process called dissolution.

Solutional

Main article: Solutional cave

Solutional caves or karst caves are the most common type of cave. They form in rocks that can dissolve, most often in limestone. They can also form in chalk, dolomite, marble, salt, and gypsum. Except for salt caves, these caves form when rock dissolves in natural acid found in groundwater that seeps through cracks in the rock. Over time, these cracks grow larger and become caves.

The biggest solutional caves are in limestone. Limestone dissolves when rainwater and groundwater, which contain carbonic acid and natural organic acids, flow over it. Limestone caves often have beautiful calcium carbonate formations made very slowly by precipitation. These include flowstones, stalactites, stalagmites, helictites, soda straws, and columns.

Primary

Caves formed at the same time as the rock around them are called primary caves.

Lava tubes

Lava tubes are formed by volcanic activity and are the most common primary caves. When hot lava flows downhill, the surface cools and hardens. This hard crust keeps the hot lava inside flowing. If the lava flows away, a hollow tube is left behind. Lava tubes can be found in the Canary Islands, Jeju-do, Eastern Idaho, and other places. Kazumura Cave near Hilo, Hawaii is a very long and deep lava tube; it is 65.6 km long (40.8 mi).

Lava caves also include other types formed by volcanic activity, such as rifts, lava molds, open vertical conduits, and inflationary blisters.

Sea or littoral

Main article: Sea cave

Sea caves are found along coasts all over the world. Littoral caves are a special type formed by waves acting on weak spots in sea cliffs. These weak spots can be faults, dykes, or places where layers of rock meet. Some sea caves are now above water because the land rose later. In places like Thailand's Phang Nga Bay, some solutional caves have been flooded by the sea and are now shaped by waves. Sea caves are usually between 5 to 50 metres (16 to 164 ft) long, but some can be longer than 300 metres (980 ft).

Erosional

Erosional caves form when streams carrying rocks and sediment erode through rock. They can form in any kind of rock, including hard rocks like granite. Usually there needs to be a weak spot, like a crack or fault, to guide the water.

Glacier

Main article: Glacier cave

Glacier caves form when ice melts and water flows within and under glaciers. The slow movement of the ice can cause the caves to collapse.

Fracture

Fracture caves form when layers of minerals that dissolve easily, like gypsum, are between layers of rock that do not dissolve. The rocks break apart and fall into blocks.

Talus

Main article: Talus cave

Talus caves form in the spaces among large boulders that have fallen down a slope, often at the base of cliffs. These piles of rocks are called talus or scree and can shift frequently due to rockfalls and landslides.

Anchialine

Main article: Anchialine cave

Anchialine caves are usually found near coasts and contain a mix of fresh water and salt water (usually sea water). They are found in many parts of the world and often have unique animals that live only there.

Physical patterns

Caves can form in different patterns depending on how water and rock interact. Branchwork caves are the most common type. They look like tiny rivers on the surface, with passages joining together like streams. These caves usually form near places where water goes into the ground.

Other types of caves include angular network caves, which have narrow, straight passages formed by cracks in the rock. Anastomotic caves look like braided streams, with passages that split and then join again. Spongework caves have a random, sponge-like pattern of cavities. Ramiform caves feature large, irregular rooms created by rising water. Finally, pit caves are vertical shafts instead of horizontal passages.

Geographic distribution

Caves are found all around the world. We know more about them in places where people have explored a lot. Countries like France, Italy, Australia, the UK, and the United States have many famous caves. These caves are mostly in Europe, Asia, North America, and Oceania. There are fewer known caves in South America, Africa, and Antarctica.

Even in these areas, the number of known caves changes. Some large parts of North America and Asia don’t have many recorded caves, while places like the Madagascar dry deciduous forests and parts of Brazil have many. As more people explore rocky areas, we may find many more caves in places we haven’t looked at closely yet. For example, China has lots of rock that could form caves, but we still don’t know about many of them.

Records and superlatives

Some caves are famous for being very long, deep, or large. The longest cave system ever explored is Mammoth Cave in Kentucky, US, stretching for over 675 kilometers. Another amazing cave is Sistema Ox Bel Ha in Yucatán, Mexico, which is the longest underwater cave known.

The deepest cave ever found is Veryovkina Cave in the Republic of Abkhazia, diving more than 2,200 meters below its entrance. The biggest room inside a cave is in Son Doong Cave in Vietnam, which has a passage that is nearly 5 kilometers long!

Ecology

Main article: Biospeleology

Caves are special places where unique animals live. Some animals can only live in caves, called troglobites. They often look different from other animals. They might be pale or white because they don’t need colour, and they may not have eyes because there’s no light underground. These animals can sense vibrations very well to find food and each other.

Some cave animals, like bats, visit caves but live outside too. Others, like certain insects and small shrimp, spend part of their lives in caves. Because caves are very delicate environments, many of these animals are rare and need protection.

Archaeological and cultural importance

See also: Underground living

People have used caves for a very long time. Some of the oldest human remains found on Earth come from caves in South Africa. These remains show us that early humans lived in caves.

Caves have also been important places for early humans all over the world. For example, in China, Indonesia, Europe, and South Africa, early humans used caves for shelter and sometimes for art. One of the oldest known art sites is in a cave in South Africa, where humans began using sea caves about 180,000 years ago.

We have found some of the world’s oldest cave paintings in many places. These paintings show that humans have been creating art in caves for tens of thousands of years.

Caves and acoustics

Caves have been important for sound for a very long time. Ancient people in Spain and France made paintings in cave areas that had special sounds. Some of these paintings showed animals, and the sounds in these spots could copy the animals' calls. People might have also used their voices to find their way in dark parts of the caves.

Today, caves are still used for music. Cumberland Caverns is a great example, where musicians enjoy the special sounds the cave makes. The unusual shapes of the cave walls help spread out sounds evenly, making it feel like a music studio. In the past, famous musicians performed in caves like Luray Caverns in Virginia, where a huge instrument called the "Great Stalacpipe Organ" creates different musical notes.