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Wind Cave is currently the 6th longest cave in the world at >118 miles long. There are many caves present in the Black Hill's Pahasapa Limestone.
This is a picture of Boxwork, one of the rarest speleothems on(under) Earth. Wind Cave has ~95% of the world's known amount of boxwork. The thin calcite fins were actually formed before the cave. Briefly explained, a long, long(long, long) time ago, there were cracks in the Pahasapa Limestone. These cracks got filled with gypsum, which later was replaced by the calcite. Once the cave started forming, the limestone was disolved while the fins were left behind.
It's truely amazing stuff. I have only seen it in one other cave outside of the blackhills.
for scale, this picture is ~ 2m across.