If you don't get out of your car at Wind Cave, you haven't scratched the surface—literally. The park has more than 145 miles of underground passageways. Curious cave formations include 95% of the world's mineral boxwork, and gypsum beard so sensitive it reacts to the heat of a lamp. This underground wilderness is part of a giant limestone labyrinth beneath the Black Hills. Wind Cave ranks as the sixth-longest cave in the world, but experts believe 95% of it has yet to be mapped.