Jewel Cave National Monument, National monument in Custer County, South Dakota, United States.

Jewel Cave National Monument protects a solutional cave formed mainly in the Pahasapa limestone of the Black Hills. Its mapped passages total 220.33 miles (354.59 km) as of July 2025, making it the fifth longest cave in the world. The passages follow faults and joints linked to the Black Hills uplift, and a calcite layer lining the walls about 2.5 million years ago gave the cave its name.

Two brothers named Frank and Albert Michaud discovered the cave in 1900 while noticing cold air seeping from a small opening. The site became a national monument in 1908 and began attracting visitors shortly after.

The name comes from the calcite crystals that line many walls, which the Michaud brothers found after blasting open the entrance in 1900. Visitors on the Lantern Tour enter the passages the brothers first explored, while the Scenic Tour follows a later half-mile loop reached by a 300-foot elevator shaft.

The best way to explore the cave is to book one of the offered tours, which have different difficulty levels. Wear comfortable shoes since the paths are uneven and temperatures stay cool underground.

The cave breathes like a living organism, with air moving in and out as atmospheric pressure changes, suggesting vast undiscovered chambers. Scientists use these pressure differences to learn more about the hidden scale of the system.