A natural underground chamber is a type of geological formation that is formed by the erosion of rocks and other materials over time. The two most common types of natural underground chambers are caves and caverns. A CAVE is a natural underground chamber that is typically formed by the dissolution of soluble rocks such as limestone, dolomite, and gypsum. A CAVERN is a larger natural underground chamber that is typically formed by the erosion of rocks such as sandstone and limestone.