So imagine this: You're strolling along a road in the Andes when suddenly you come across a patch of very spiky ice - spikes that hit as high as your waist, as high as your shoulders. There isn't any other snow around, just rocks and gravel... and these strange spikes of ice.
It'd be a little weird, right? These are called Penitentes (or nieves penitentes). The tall, thin spikes are typically blade-shaped and can be found at high altitudes above 13,000 feet and are made of hardened snow or ice. The blades usually are oriented toward the sun.
When Charles Darwin first described these strange formations in 1839 after seeing them near the Piuquenes Pass between Chile and Argentina, the local belief was that they are formed by the strong winds of the Andes. However, they're a bit more complicated than that. From Wikipedia:
Louis Lliboutry noted that the key climatic condition for the differential ablation that leads to the formation of penitentes is that the dew point is always below freezing. Thus, snow will sublimate, because sublimation requires a higher energy input than melting. Once the process of differential ablation starts, the surface geometry of the evolving penitente produces a positive feedback mechanism, and radiation is trapped by multiple reflections between the walls. The hollows become almost a black body for radiation, while decreased wind leads to air saturation, increasing dew point temperature and the onset of melting. In this way peaks, where mass loss is only due to sublimation, will remain, as well as the steep walls, which intercept only a minimum of solar radiation. In the troughs, the ablation is enhanced, leading to a downward growth of penitentes.
In 2006, researchers were able to create penitentes in the lab, a first. The American Physical Society reports, "After some false starts, the apparatus that worked was a large horizontal freezer with a clear Plexiglas top. The team cooled the air by sending it through liquid nitrogen and also controlled humidity. They shined a flood lamp onto a block of snow or ice in the freezer and produced one- to five-centimeter-tall spikes on the block within a few hours."
"Understanding their growth may give researchers insight into the process of glacier “evaporation” and may shed light on similarly-shaped micro-spikes that appear in materials like those in solar-powered energy cells," states the researchers. It may also help them in understanding how to preserve glaciers in a warming world.