// HACKER NEWS — CYBERSECURITY
The Mystery of Dark Oxygen
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.
The scientists had come not for precious metals but for something even more elusive: what they call dark oxygen. On Earth’s surface, plants and algae produce plentiful oxygen from sunlight and water, allowing us to live and breathe. At one time, scientists didn’t think that significant quantities of free oxygen, or O2, would be found anywhere else on the planet, in part because O2 reacts aggressively with other elements. “O2 is way less abundant in the universe than diamond,” Emil Ruff, a microbial ecologist and the expedition lead for the trip, told me. But, over the past few years, small quantities of free oxygen have been detected in a range of deep places, far removed from the light of the sun. Ruff and his colleagues hoped to collect new samples of ancient water in the mine. They suspected that oxygen was playing an unrecognized role in powering underground life.
We were issued boots, helmets, and reflective coveralls strongly reminiscent of the music video for the Beastie Boys song “Intergalactic.” So costumed, we passed through another checkpoint, into a room filled with headlamps and emergency oxygen packs. A poster on the wall explained what to do in the event of a cave-in: pull a tab to inflate a bag with a few crucial minutes of breathable air. “We’re embarrassingly dependent on oxygen,” Karen Lloyd, a University of Southern California biogeochemist, who was there to research how underground microbes respond to seismic activity, said. Many of the subsurface microorganisms she studies don’t breathe oxygen, as we do, instead using sulfur or iron compounds that leach from the rock to drive their metabolism.
Once everyone was suited up, we clambered into a three-story-tall lift called the Cage, which ferries thousands of workers up and down the shaft each day. I tried not to think about the void—seven times the height of the Empire State Building—beneath us. If we fell in, I calculated, a full minute would pass before we hit the bottom. The operator pulled down a gate with a clang, plunging us into darkness. We began to drop.
Enterprising scientists have been conducting research in Moab Khotsong for many years, but the mine became particularly valuable to biologists after an earthquake struck nearby in 2014. A group of Japanese geoscientists soon started drilling toward the fault line under the shaft. After half a mile, extremely salty radioactive water started flowing out of the borehole. This water, in turn, attracted researchers who investigate the deep biosphere—the poorly understood microbial ecosystems in the earth’s crust—which has been estimated to contain ten to fifty per cent of the biomass on Earth. Tullis C. Onstott, the late Princeton geobiologist who authored “Deep Life: The Hunt for the Hidden Biology of Earth, Mars, and Beyond,” told colleagues that he’d spent twenty-five years searching for water like what was found in Moab Khotsong. “It’s totally separate from the surface world,” Thomas Kieft, a microbiologist who worked with Onstott, told me. “It’s totally separate from photosynthesis.”
Geochemical measurements suggested that the water had been separated from the surface for at least 1.2 billion years—more than a quarter of Earth’s history, since before animals and pl