In 2013, Lee Berger, a National Geographic Explorer-in-Residence, caught wind of a cache of bones in a hard-to-reach underground cave in South Africa. He put out a call around the world for petite collaborators--men and women small and adventurous enough to be able to squeeze through 8-inch tunnels to reach a sunless cave 40 feet underground. With this team of "underground astronauts," Berger made the discovery of a lifetime: hundreds of prehistoric bones, including entire skeletons of at least 15 individuals, all perhaps two million years old. Their features combined those of known prehominids like Lucy, the famous Australopithecus, with those more human than anything ever before seen in prehistoric remains. Berger's team had discovered an all new species, and they called it Homo naledi.