It’s not easy making a living among the wind-scoured rock faces of the Himalaya, home to the world’s tallest mountains. Beyond large boulders and caves, only a few scraggly trees provide protection from predators and shrieking gusts.

Among the hardy inhabitants is the woolly flying squirrel (Eupetaurus cinereus), which, at five pounds and three feet long, is one of the world’s largest squirrels. It’s also one of the least known mammals on Earth: First named 130 years ago, the house cat-size rodent was believed extinct until its “rediscovery” in the 1990s.  

Intrigued by some recent squirrel sightings in the Himalaya, Helgen—a National Geographic Explorer—and his colleagues decided to dig deeper into this mysterious species by examining museum