A Baikal seal (Pusa sibirica) dives beneath the thick ice that blankets Russia’s Lake Baikal in winter. This is the world’s only completely freshwater seal, and it’s found nowhere else. How the species established a home in this place—the oldest, deepest, and most voluminous mass of freshwater in the world—remains a mystery. Experts hypothesize that members of the species may have traveled up ice-choked rivers from the Arctic Ocean during the last ice age. Today, Baikal seals use their claws to scrape breathing holes in ice that, in winter, can be up to two meters thick. As challenging as these conditions might seem, the greatest threat that Baikal seal populations face is pollution from factories situated along the lake’s shores.