If you'll only visit one Black Hills cave, this would be a good choice. It's 13 miles west of Custer and is so named because calcite crystals line nearly all of its walls. Currently 145 miles have been surveyed, making it the second-longest known cave in the world, although it is presumed to be the longest.
The visitor center has useful exhibits. Tours vary in length and difficulty and are offered on a first-come basis.
