The Mystery of Dark Oxygen
Scientists once thought that life-sustaining oxygen existed only near Earth’s surface. Then they detected it deep underground.
In March, I accompanied an international team of scientists on an expedition to one of the deepest places that humans have ever visited, a few hours outside Johannesburg, South Africa. Before sunrise, we drove to the entrance of the Moab Khotsong gold-and-uranium mine, joining a line of workers who were relieving the night shift. Security was tight; for years, mining companies have struggled to keep out illegal miners called zama-zamas, who sneak underground and collect ore at great danger to themselves. When guards finally cleared us and our gear, we walked through rotating gates into what resembled a suburban office park. At its center was a boxy concrete tower—a mine shaft that would carry us nearly two miles down into the earth.