TRAVEL + LEISURE World's Coolest Underground Wonders

Cave of the Crystals, Mexico

Carsten Peter/Speleoresearch & Films/National Geographic Stock

La Cueva de los Cristales was discovered in the Naica Mine near Chihuahua in 2000 after water was pumped out of the 30-by-90-foot chamber, and there’s nothing else like it on—or under—earth. The crisscrossing gypsum columns are some of the world’s largest natural crystals. Despite its grandeur, visits are nearly impossible to come by due to dangerous conditions: near 100-percent humidity and temperatures as high as 136 degrees, warmed by a pool of magma sitting below the cave. There’s even been talk of refilling it with water.