Watermelon snow is snow which has become discolored due to the presence of cold-loving algae, specifically Chlamydomonas nivalis, an alga with a distinctive pink to red color. When this organism colonizes a section of snow, the snow appears pinkish to red, depending on how compressed it is, and it even has a faint watermelon scent. Watermelon snow tends to appear at high altitude, and it can be found all over the world, including regions where snow appears seasonally, rather than year-round.
People have been noticing watermelon snow for thousands of years. The Ancient Greeks, for example, puzzled over the phenomenon, as did 19th century explorers in the New World, who concluded that the strange red color was due to the presence of iron. Only with the assistance of microscopes did people realized that watermelon snow was caused by a living microorganism. The existence of cold-loving algae also suggested that other extremophilic organisms might someday be discovered, and this proved to be the case, illustrating that life will thrive just about anywhere, if given a chance to do so.
The red color isn't just for looks. The carotenoid pigment which turns the algae pink helps to insulate it from the cold, and to protect it from harmful UV radiation. The evidence seems to suggest that the algae actually colonizes the ground, and when it becomes covered with snow, it slowly works its way to the surface, creating streaks, pits, and patches of reddish color in the snow. Watermelon snow can sometimes extend across a very large area, creating a very striking sight.
When people walk on watermelon snow, they compress the algae, resulting in a deeper red color. They also pick up the color on their shoes and pantlegs, leaving a trail of pink footprints in the snow until all of the algae has been scuffed off their garments and shoes. Since watermelon snow is so visually distinctive, it often becomes a topic of conversation on hikes, not least because it looks like the ghoulish remains of a climbing accident.
Technically, watermelon snow is edible. However, snow can easily be contaminated with bacteria and algae which are not safe to eat, and eating large quantities of watermelon snow can lead to intestinal distress. Therefore, eating this discolored snow is not recommended, although people interested in the science might want to collect a small sample to examine under a microscope at home. Magnified, the algae which causes the peculiar red color is actually quite pretty.