In a nearly empty cantina in a dark desert town, the short, drunk man makes his pitch. Beside him on the billiards table sits a chunk of rock the size of home plate. Dozens of purple and white crystals push up from it like shards of glass. "Yours for $300," he says. "No? One hundred. A steal!" The three or four other patrons glance past their beers, thinking it over: Should they offer their crystals too? Rock dust on the green felt, cowboy ballads on the jukebox. Above the bar, a sign reads, "Happy Hour: 8 a.m. to 9 p.m."

This remote part of northern Mexico, an hour or so south of Chihuahua, is famous for crystals, and paychecks at the local lead and silver mine, where almost everyone works, are meager enough to inspire a black market. "Thirty dollars." He leans in. "Ten." It's hard to take him seriously. Earlier in the day, in a cave deep below the bar, I crawled among the world's largest crystals, a forest of them, broad and thick, some more than 30 feet long and half a million years old. So clear, so luminous, they seemed extraterrestrial. They make the chunk on the pool table seem dull as a paperweight.  Nothing compares with the giants found in Cueva de los Cristales, or Cave of Crystals. The limestone cavern and its glittering beams were discovered in 2000 by a pair of brothers drilling nearly a thousand feet below ground in the Naica mine, one of Mexico's most productive, yielding tons of lead and silver each year. The brothers were astonished by their find, but it was not without precedent. The geologic processes that create lead and silver also provide raw materials for crystals, and at Naica, miners had hammered into chambers of impressive, though much smaller, crystals before. But as news spread of the massive crystals' discovery, the question confronting scientists became: How did they grow so big?

It takes 20 minutes to get to the cave entrance by van through a winding mine shaft. A screen drops from the van's ceiling and Michael Jackson videos play, a feature designed to entertain visitors as they descend into darkness and heat. In many caves and mines the temperature remains constant and cool, but the Naica mine gets hotter with depth because it lies above an intrusion of magma about a mile below the surface. Within the cave itself, the temperature leaps to 112 degrees Fahrenheit with 90 to 100 percent humidity—hot enough that each visit carries the risk of heatstroke. By the time we reach the entrance, everyone glistens with sweat.