What causes these spooky ice formations in the desert?

The Atacama Pathfinder Experiment telescope, or APEX, is on Earth, but surrounded by ice spikes that make the landscape look decidedly alien. These "sculptures" are natural formations known as penitentes. And they form where it's very cold and very dry.

Many people have heard of the Atacama desert - a place where it is so dry in places that footprints in the dust stay undisturbed for decades. The dry air and the lack of light pollution make it perfect for stargazing, which is why the APEX telescope got built out there. In this photo, it's gathered its own little crowd of onlookers. These ice formations are called penitentes, after a local religious order whose members wear spiked white hoods.

Naturally, there needs to be a little moisture around to make a field of snow, but the penitentes form because the climate is so dry. The snow undergoes a process of sublimation - when ice changes directly to gas without changing to water first. The sublimation causes pits in snow that hollow out to deeper caverns. When ice sublimates in the caverns, it turns to mist - and makes the climate in the cavern wet enough for liquid to form. The combination of melting and sublimation sculpts elaborate ice figures, which look even cooler next to a telescope.

Via Spiked Ice.