"Ice scrapes the earth as if it had claws." —Gretel Ehrlich, The Future of Ice, 2004

IN THE MIDST OF A SWIRLING SNOWSTORM at 14,000 feet on Mt. Aconcagua, Argentina, my climbing party encounters the first outpost of penitentes. Jagged and fluted, with sharp and narrow peaks, these formations emerge like a little Manhattan of ice, with skyscrapers as tall as our heads.

As we climb higher on the mountain, we find tens of thousands of these spikes armoring the slopes. Penitentes are so named because they resemble processions of religious penitents who wear robes with high, pointed hoods and demonstrate their faith by whipping themselves. The name seems particularly apt as I struggle through a steep obstacle course of ice. Between my gasps for air, I think of Charles Darwin's first sighting of penitentes: Sticking up among them were the hind legs of a dead horse that had likely plunged headfirst into a cleft.

Penitentes form when solar rays hit a field of consolidated snow. In most mountainous regions this creates the shallow hollows known as sun cups. But here in the southern Andes, where penitentes are more numerous than anywhere else, the air is so dry and the sunshine so intense that the snow in the hollows doesn't melt but vaporizes, a process known as sublimation. As the cavities deepen, the sunlight reflects off their walls, creating a solar oven that intensifies the effect until the depressions bore to the ground. By midsummer the ice pinnacles tower up to 12 feet tall.

In 2006, scientists discovered how to create penitentes in a lab and suggested how they might help alleviate our global-warming problems. Because the shade cast by penitentes seems to slow their melting, seeding Andean snowfields with tiny bits of sediment might "grow" more ice spires and thus curb the disappearance of the glaciers that provide a crucial source of water for Chile and Argentina. Also, the heat-absorbing peaks and valleys might inspire a new generation of more efficient solar cells. Perhaps these ghostly sentinels will guard not just access to the high peaks and glaciers but their very existence. —David Ferris