A cave (or cavern) is a naturally occurring area or space under the surface of the Earth. Caves are often a system of interconnected passageways created by the weathering of rock.

Recreational or scientific exploration of a cave system is called caving, potholing or spelunking.

The scientific study of caves and their surrounding environments is called Speleology and the formation and development of caves called speleogenesis.

The maximum depth a cave system can reach underground before the pressure of overlying rocks becomes too great is estimated to be about 3,000 m (9,800 ft).

Caves can be formed by many geological processes including chemical actions, erosion from water, tectonic and volcanic forces, microorganisms, and pressure.

The most common type of cave formation are solutional caves which are formed by natural acids in groundwater dissolving soluble rock such as limestone (or sometimes chalk, dolomite, marble or gypsum).

Primary caves are formed at the same time as the surrounding rock, the most common type are lava tubes. As lava flows downhill the surface cools and hardens, yet underneath, hot lava continues to flow leaving behind a hollow tube.

Sea caves (or littoral caves) are located on the coast and are created by continual wave and tidal action which erodes or weakens sea cliffs.

Corrasional caves (or erosional caves) are formed by erosion as sediment laden streams flow through rock. These caves can form in any rock type including very hard granites. They usually require an initial fault or joint in the rock through which the water can flow.

Glacier caves are formed as melting ice creates a flow of water within the glacier.

A fracture cave is created by the dissolving of more soluble mineral rocks sitting between layers of less soluble rock, creating fracture collapses.

Stalactites are created by the continuous dripping of mineral rich water, which little by little leaves mineral deposits on the ceiling as the minerals harden.

Stalagmites are created as the mineral rich water droplets fall to the cave floor leaving mineral deposits that grow to become pillar-like formations.

Stalactites and stalagmites can eventually join from floor-to-ceiling, however they grow very slowly, around an inch every 100 years.

The world's three longest known cave systems are: Mammoth Cave, Kentucky, USA, 651.8 km (405 mi), Sistema Sac Actun/Sistema Dos Ojos, Mexico, 319 km (198.2 mi), and Jewel Cave, South Dakota, USA, 267.6 km (166.3 mi).

The deepest known cave is Voronya Cave in Georgia, at 2,197 m (7,208 ft).

Caves have been used throughout human history as shelter, for burials, and as religious sites. Many archaeological treasures and ancient paintings have been found in caves around the world.

Cave inhabiting animals include bats, fish, salamanders, insects and spiders.

Troglobites are a type of animal that only live within cave ecosystems, they often have some unique characteristic such as a loss of pigment coloring and no eyesight or eyes which developed due to the extreme darkness.