Jewel Cave, Show cave in Custer County, United States

Jewel Cave is a show cave in Custer County featuring more than 330 kilometers of explored underground passages. The cave impresses visitors with its shimmering calcite crystal formations that sparkle in the light.

Two gold prospectors named Frank and Albert Michaud discovered the cave in 1900 while investigating wind sounds from a rock opening. This accidental find led to decades of exploration that continues today.

The cave's name comes from the calcite crystals that line much of its walls, which the Michaud brothers took for precious stone. The National Park Service runs three guided tours: a paved, lit half-mile route reached by elevator, a candlelit historical route, and a spelunking tour into an undeveloped section.

The cave is open year-round. The Park Service offers three tours: the Scenic Tour, a half-mile route with a paved, lit central section reached by elevator; the Historic Tour, lit by candle through the gallery; and the Spelunking Tour, through an undeveloped area near the Scenic Tour route.

Air-flow measurements show that the explored passages hold less than 5 percent of the cave's estimated air volume, a figure calculated by comparing how much air the cave exhales when outside pressure drops and inhales when it rises. The 2021 survey length of 346.09 km makes it the world's fifth-longest cave.