Caves are natural underground spaces large enough to contain passages or chambers. They occur in several kinds of rock and can form through different processes. Some develop as water gradually enlarges openings, while others are associated with lava, waves, or the movement of material. A cave entrance provides only a small view of a system that may extend in unexpected directions beneath the surrounding ground.

In soluble rocks, water can play a major part in cave development. It moves through cracks and gradually changes the material along its route. Over long periods, small openings may become larger passages. The path followed by the water depends on the arrangement of the rock and on differences in height. Underground streams can therefore be connected with springs and drainage patterns that are visible at the surface.

Some caves contain deposits that grow as minerals are left behind by dripping or flowing water. Hanging forms, rising forms, and thin layers on walls may develop under suitable conditions. Their appearance reflects the movement and chemistry of the water, as well as time. Such deposits are not present in every cave, and they are different from the processes that originally created the open space around them.

Conditions within a cave differ from those outside. Light diminishes away from the entrance, and temperature or moisture can vary less than at the surface. These conditions influence the organisms able to live there. Material brought by water, air, or visiting animals can provide resources in a place with little direct sunlight. The connection between the cave and the outside landscape remains important even when the surroundings appear isolated.

Understanding a cave involves considering its shape, the rock, signs of flowing water, and changes along the route from the entrance. Surface features can provide clues about what happens underground. A passage that is dry at one time may carry water under other conditions. Caves show that a landscape has depth as well as a visible surface, with connected spaces and processes that are only partly revealed from above.