A cave is a space dug in a soil, on a cliff, or in a hillock, especially a natural basement with a hole leading to the surface of the ground. It is usually formed by the erosion of water or the weathering of other external forces such as wind and microorganisms. The most popular method of cave classification is to divide by scale. The performance of a caver is also reflected by the length and depth of the cave. Water and soluble limestone are prerequisites for the formation of karst caves. Water dissolves the soluble bedrock into calcium bicarbonate. Year after year, the space of the tunnel increases continuously. When it can't bear the top weight, it collapses. The collapse only changes the shape of the space and does not change the size of the space. The collapsed material is brought along with the water flow, and the water flow continues to dissolve the bedrock. So repeatedly, the cave system is formed. The following chemical reaction formula is the principle of cave formation, but it is reversible.