Jewel Cave National Monument is more than 166 miles of underground labyrinth, named for its calcite crystal walls. On the Wild Caving Tour, you trek through an 8.5-inch crawl space and up 90-degree chutes. But the most intense part is Hurricane Corner, where a 35-mile-per-hour barometric wind screams through the blackness.

Getting there: From the airport in Rapid City, drive 50 miles to the $27 Wild Caving Tour, open from the end of May to the end of October and monthly during the off-season.