Caves are natural voids in the earth, most often formed by the dissolution of limestone or dolomite rocks. They can also be created through volcanic activity, faulting, erosion, and other processes. Caves occur in a wide range of sizes and shapes, from small pockets less than a meter in diameter to large chambers several kilometers in length. Most caves are relatively dark due to their lack of surface openings, although some do have skylights or other means of admitting light.
The temperature inside a cave is usually fairly constant throughout the year and remains cool even in hot climates. This makes caves an ideal environment for many types of animals that cannot tolerate extreme temperatures. In fact, some caves are so stable that they have been used as human dwellings for thousands of years.
The study of caves is known as speleology (from the Greek word σπήλαιος, spī́laion, meaning “cave”). Speleologists investigate all aspects of caves and their development, including their geologic origins, morphology (shape), hydrology (water resources), climate history, biology (inhabitants), paleontology (fossils), and archaeology (artifacts).