This national monument is the world's third-longest cave and might move up the list to the top spot if cavers keep discovering new passages along its 164-mile maze. Jewel Cave doesn't wow with stalagmites or stalactites; the sparkle is what draws most visitors. Much of this showy cave resembles a giant geode: rough on the outside, streaked with white crystals and full of glistening surprises inside. The "jewels" are actually calcite; nailhead calcite composes the nubby, rough walls that look like coral; dogtooth spar provides the pockets of pretty crystals. A sacrificial rock at the beginning of the tour lets visitors feel the cut-glass texture of the stones. One of the defining features looks nothing like a jewel, but is instead called bacon, a wavy, amber-stripe formation close to four feet long and beautifully backlit.
Elevators zip guests about 300 feet down into the 49-degree cavern (be prepared for a humid, moldy smell when you first step out of the doors that quickly disappears once you move into the cave). Ranger-guided tour options range from a 20-minute Discovery ($4 adult admission) to hours-long strenous get-muddy Wild Caving adventures ($27, age 16 and over only). Each tour builds in time to gape at the sights, take photos and briefly experience absolute darkness when all the lights go out.