How Caves are Formed Caves are underground passages that go deep inside the Earth. Caves are created through a combination of chemicals, water and pressure. The majority of caves are found in limestone areas. Limestone is a very soft rock, so water can easily shape or carve out a cave. When it rains, water enters the soil and mixes with it. When the water comes through the soil, the oxygen(or carbon dioxide) creates an acid called carbonic acid, helping to dissolve the one. But sometimes water only drips a little bit, trickling bit by bit. This geologic process gradually eats away at the limestone and creates an underground hole. Over time, the hole becomes larger and larger. Over many thousands of years, the cave gradually gets bigger. Eventually it becomes big enough to walk inside and explore.