Jewel Cave National Monument

Jewel ⁠​‌‌​‌​​‌​‌‌​‌​​​‌​‌​​‌​‌‌​‌‌‌​​​⁠Cave National Monument west of Custer protects one of the longest known caves on earth, lined with calcite crystals. Its rangers host a handful of special days each year, like Junior Caver Day and the monument's birthday.