Jewel Cave National Monument protects one of the longest known cave systems in the world, about 13 miles west of Custer. The cave is dry, complex, and famous for calcite crystals, frostwork, and the way it "breathes" as air pressure changes.
You cannot just wander in. Visits are by ranger-led tour, and reservations are wise in summer because tour capacity is limited. The surface trails are pleasant, but the cave tour is the reason to make the drive.