Caves can be found all over the world. Some caves are open to the public, while others are wild and untouched. Sometimes called caverns, caves are natural hollows in the Earth, usually created by water or wind. Prehistoric humans sometimes lived in caves. Archaeologists have found many ancient artefacts and paintings in caves.
Most plants need sunlight to grow, but some plants, bacteria, and fungi use the energy from decaying matter instead of light. These can be found in the darkest caves.
A variety of animals make caves their homes, some seasonally, others permanently. Bears, raccoons, and other animals nest and shelter in caves. Many species of bats live in caves. Cave floors are often slippery with guano (bat droppings). Bats and other animals that leave the cave for food are known as trogloxenes.
Animals that always live in the darkest parts of a cave are called troglobites. Some salamanders, fish, insects, and spiders are troglobites. Many of these animals are blind and have no pigmentation to their skin, appearing transparent.
Caves take many thousands of years to form; some are 50 million years old. As long as a cave has water, it is considered living and ever-changing. A cave without water does not change and is considered dead. Some caves have areas with and without water.
Scientists who study caves are called speleologists. Amateur cave explorers are called spelunkers; these adventurers enjoy testing their strength, skill, and nerve while exploring caves.
Limestone caves have rock formations called speleothems on the walls, floor, and ceiling. Stalactites hang like icicles from the ceiling, and stalagmites grow up from the ground. These formations are created by calcite deposits. Speleothems often have other minerals in them that make them different.