From boiling heat to bone-chilling cold, from the driest deserts to the darkest caves, from the tallest mountains to the deepest seas: these animals survive in the most extreme environments imaginable. Meet these incredible creatures, including polar bears and penguins that endure freezing temperatures, brine shrimp that swim in salty waters, birds that fly higher than airplanes, and even the ancient microscopic "water bear," an "extremophile" that could stay alive in outer space!