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. Some 187 miles have been surveyed so far (3% of the estimated total), making it the third-longest known cave in the world. Tours vary in length and difficulty and can be reserved three to 90 days in advance (605-717-7629, www.blackhillsvacations.com). The visitor center has useful exhibits.