Cave

Caves are underground cavities connected to the surface by an entrance. They occur in many kinds of rock, but are especially common in limestone. In these rocks, rainwater absorbs carbon dioxide from the soil and becomes slightly acidic. As it seeps through cracks, it can slowly dissolve the limestone and enlarge the passages.

Over long periods, cracks may develop into chambers, galleries, shafts and underground networks. Other caves form through different processes: lava tubes can remain after molten rock flows away, while waves may carve cavities into coastal cliffs. Groundwater, ice and erosion can also contribute to cave formation in some environments.

Inside a cave, conditions are often different from those at the surface. Light disappears beyond the entrance, temperatures usually vary less, and humidity can be high. In complete darkness, green plants cannot grow because they need light for photosynthesis. Even so, caves can support specialized organisms, including bacteria, fungi and animals adapted to darkness.

Some species spend their entire lives underground, while others use caves for shelter or reproduction. Water moving through caves may deposit minerals that form stalactites hanging from ceilings and stalagmites rising from floors. Because such formations can take a very long time to grow, caves are environments that require careful protection.