Featured in today's image—the polar bear—is the largest living species of bear, often weighing over 180 kilograms. Closely related to brown bears, polar bears have evolved to survive in the planet's harshest, coldest environments, perfectly adapted to life on snow and ice.

The town of Churchill, in Manitoba, Canada, is one of the best places to see them. Hundreds of polar bears gather nearby, waiting for Hudson Bay to freeze so they can hunt seals again—their primary food source. Ever wonder how they stay warm? A thick layer of fat and a dense undercoat trap heat. Their tangle-resistant hairs shake dry after swimming and natural oils stop the coat from freezing, keeping these Arctic giants well insulated against the chill. But even these masters of the ice face mounting challenges. As climate change melts their sea-ice habitat, polar bears have been listed as threatened under the US Endangered Species Act since 2008.