Wind Cave, named for the revolving air exchange between the cave system and atmosphere, is noted for its boxwork, popcorn and frostwork formations as well as its many miles of maze-like passageways. Not only is this 132-mile (213 km) cave the third longest cave in the U.S. and fourth longest in the world it boasts 95% of the world's thin calcite fins called boxwork. Yet what makes Wind Cave National Park a true gem is the geology of the maze-cave system and the environment above. Limestone, gypsum (calcium sulfate), acid-rich water and lots of time - 300 million years give or take a few - have all contributed to the formation of this complex and unusual cave. Up top the cave sits a whole 'nother world. Wind Cave National Park is one of the few remaining mixed-grass prairies in the United States. The natural entrance of the cave, once referred to as the hole that blew air, is a sacred site of the Lakota Indians. What's more, the park is home to native wildlife such as bison, elk, pronghorn, mule deer, coyotes and prairie dogs.