All around the world, in the cold, people walking near high-altitude rivers and lakes see streaks of vibrant "watermelon snow." This bright pink snow can color deep snow wells, or make cliffs look like they are streaked with blood. Find out what makes snow pink.

Deep bloody holes have been seen in snow in Scotland and Greenland. Candy cane stripes streak down mountains in the Sierra Nevada. Red leaks into pools of new-melted snow all over the world in a phenomenon that's come to be known as watermelon snow.

These pink lines are the result of a kind of extremophile algae. Chlamydomonas nivalis is a type of green algae, but it has a deep red pigment that makes it pink, or even red when it gathers in high concentrations. The red color is an adaptation to life in the snow, which is C nivalis' preferred habitat. The snow around it reflects light, including ultraviolet light, so it gets a double dose of radiation. Its red color acts as natural sunscreen, allowing it to minimize the damage from the strong light. The red pigmentation also helps it garner whatever warmth it can get from the surrounding area.

You're most likely to see watermelon snow in the summer, high in the mountains where the snow melts late in the year. Or in the winter, at crime scenes.

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[Via Surface gas-exchange properties of snow algae, Don't eat the pink snow]