They had found what looked like an ancient thigh bone in a completely different cave. "Can we go get it?" they asked.

Berger was overseeing a team of 60 people, some of whom were 18 meters below ground gathering fossils. "This was day two. Lives were in danger. This was the beginning of my hair turning really white," says Berger, of the University of the Witwatersrand in Johannesburg, South Africa. "I said 'No, and don't tell anyone. I don't want anyone distracted.'" 

But on the last day of the expedition, which retrieved 1500 fossils of a mysterious new species of hominin named Homo naledi, Berger gave the spelunkers the go-ahead. They came back with the thigh bone plus photos of a skull poking out of the dirt in a second chamber of the cave system. "I couldn't believe it," Berger says.