If you visit only 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 much of its walls. Currently 187 miles have been surveyed (3% of the estimated total), making it the third-longest known cave in the world.
The visitor center has useful exhibits. Tours vary in length and difficulty (including the strenuous Wild Caving tour) and are offered on a first-come, first-served basis.