Fodor's Expert Review Jewel Cave National Monument

Jewel Cave's more than 200 miles of surveyed passages made it the third-longest cave in the world as of 2020, while exploration continued. But for tourists who aren't cavers, it's the rare crystalline formations that abound in the cave's passages—not the cave's size—that are the main draw. Take one of the paid, year-round, ranger-led tours, and you'll be rewarded with the sight of tiny crystal Christmas trees, hydromagnesite balloons, and delicate calcite deposits dubbed "cave popcorn." Plan to arrive early in the morning, because summertime tours fill up fast and start at prescheduled intervals. While you wait, scenic surface trails and exhibits in the visitor center can be explored for free.