Geologically speaking, the Colossal Cave system is a relative newcomer to the neighborhood. About 300 million years ago much of the American Southwest was covered by a vast sea populated by giant sharks, tetrapods and other primitive amphibians. Eventually the waters receded. Organic material left behind dried out and was compressed to form the limestone you see today. Then 80 million years ago the heaving earth thrust limestone and granite together to create new landforms such as the Rincon Mountains where Colossal Cave is located. After eons of water erosion, the Cave reached a dry state, and today Colossal Cave is one of the largest dry caves in north America — a snapshot of the formations as they were created millennia ago.