Answer: Pink

If you hit the slopes of the Sierra Nevada mountains during the summer, you might be in for a little surprise: reddish-pink snow. The snow, known as “blood snow” and “watermelon snow,” is tinged the color it is thanks to a species of green algae, Chlamydomonas nivalis, that contains a secondary red carotenoid pigment in addition to the green color of the chlorophyll we typically associate with algae.

Disturbing the snow damages the membranes of the algae and makes the color more pronounced, which means ski tracks and footprints stand out bright red compared to the lighter surrounding snow; it’s a rather startling change to new skiers and hikers in the area.

The phenomenon was first observed by Aristotle (“watermelon” snow occurs in alpine and coastal polar regions all around the world, not just in the Sierra Nevada region) and puzzled scientists for centuries thereafter. Early theories proposed that the coloration was caused by minerals, oxidation, and even dust left behind by meteoric activity. In 1818, Robert Brown theorized that it was a type of algae (a theory confirmed by later sample gathering and analysis).