The travertine terraces of Mammoth Hot Springs in Yellowstone National Park. The terraces of the Hot Springs are formed when hot water seeping into the earth comes into contact with carbon dioxide rising from the magma deep below the park. Some of this CO2 dissolves in the hot water forming a weak solution of carbonic acid. The acid in turn dissolves large amounts of limestone. When the saturated water reaches the surface, the carbon dioxide, no longer under pressure escapes the water. Unable to stay in solution, the limestone precipitate out, forming the travertine that makes up the terraces.