A small National Park at the edge of the Black Hills, Wind Cave was the first cave to be set aside for protection in the World. Underground, Wind Cave is the eighth longest known cave in the World and also the densest in the World, the "wind" being created from pressure equilibriation between the atmosphere and the cave through its many passages and small openings. It is known for its "boxwork" a unique honeycomb-like speleogen created by blade-shaped calcite deposits that form within cracks of bedrock that later wear away. While an interesting geological oddity however (95% of known boxwork residing in Wind Cave), they aren't especially impressive to look at.

Aboveground Wind Cave preserves the largest remaining natural mixed-grass prairie in the United States. I think both myself and the friend I visited with found the grassland to be more interesting, with its large population of pronghorn antelope, bison and prairie dogs in such a small area. While my friend amused himself chasing after the prairie dogs, I went up a small trail, at least until a herd of bison charged up the hill in front of me and started locking horns, blocking my passage further. As I turned back I noticed a harem of pronghorn antelope hanging out on a hill nearby. A curious bison calf also noticed the group and broke from its own herd, slowly edging closer and closer to the female pronghorns. Then a male pronghorn noticed and immediately lowered its horns and charged the bison calf, which fled crying back to its own herd.

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Grandaddy - Bison on the Plains

From  TheRoguez!