Caves are natural cavities that form in hillsides and cliff walls and underground, and are accessible at Earth's surface. There are many different types of caves, including solution caves, tectonic caves, boulder caves, sea caves, and lava tubes. All typically form over thousands of years as a result of natural mechanical and chemical processes in soft, fractured, or soluble rock. This interactive activity adapted from the National Park Service details and depicts caves and other karst features.