Water dissolves soluble rock and leaves hollow spaces behind. A cave is a natural underground chamber large enough for a person to enter. Most form in limestone through the action of slightly acidic groundwater, though lava tubes and sea caves arise by other processes.

Passages may extend for many kilometers and contain distinctive mineral formations such as stalactites and stalagmites. Caves preserve unique ecosystems adapted to permanent darkness and stable temperatures. They also serve as archives of past climate through chemical deposits and trapped sediments.

Explorers map caves with careful survey techniques. Scientists study their biology, hydrology, and paleoclimate records. Because caves are connected to surface drainage, pollution can travel underground for long distances, making protection of recharge areas important.

• Natural underground hollow accessible to humans
• Most commonly formed by limestone dissolution
• May contain speleothems and unique fauna
• Important for climate and hydrological research