Jewel Cave National Monument

West of Custer, South Dakota, beneath the floor of Hell Canyon, lies a wonderland far removed from the forest and mountains above, but still an integral part of the Black Hills. this is the maze of Jewel Cave, the fourth-longest cave system in the world, with more than 100 explored miles (160 km). Jewel Cave takes its name from glittering calcite crystals that line its walls.