The Mysterious, Deep-Dwelling Microbes That Sculpt Our Planet

In the middle of North America, there is a portal to the deep recesses of Earth’s rocky interior. The portal’s mouth — a furrowed pit about half a mile wide — spirals 1,250 feet into the ground, expos­ing a marbled mosaic of young and ancient rock: gray bands of basalt, milky veins of quartz and shimmering con­stellations of gold. Beneath the pit, some 370 miles of tunnels twist through solid rock, extending more than 1.5 miles below the surface. For 126 years, this site in Lead, S.D., housed the Homestake Mine, the deepest and most productive gold mine on the continent.

In 2006, the Barrick Gold Corporation donated the mine to the state of South Dakota, which converted it into the largest subterranean laboratory in the United States, the Sanford Underground Research Facility. Although the lowest tunnels flooded after mining ceased, it is still possible to descend nearly a mile beneath the planet’s surface. Most of the sci­entists who do so are physicists conducting highly sensitive experi­ments that must be shielded from interfering cosmic rays. But a few biologists also venture into the underground labyrinth, typically seeking its dankest and dirtiest corners — places where obscure creatures extrude metal and transfigure rock.

On a bitingly cold December morning, I followed three young sci­entists and a group of Sanford employees into “the cage” — the bare metal elevator that would take us 4,850 feet into Earth’s crust. We wore neon vests, steel-toed boots and hard hats. Strapped to our belts were per­sonal respirators, which would protect us from carbon monoxide in the event of a fire or explosion. The cage descended swiftly and sur­prisingly smoothly. Our idle chatter and laughter were just audible over the din of unspooling cables and whooshing air. After a controlled plum­met of about 10 minutes, we reached the bottom of the facility.

Our two guides, both former miners, directed us into a pair of small linked rail cars and drove us through a series of narrow tunnels. Within 20 minutes, we had traded the relatively cool and well-ventilated region near the cage for an increasingly hot and muggy corridor. Whereas the surface world was snowy and well below freez­ing, a mile down it was about 90 degrees with nearly 100 percent humidity. Heat seemed to pulse through the rock surrounding us, and the air was thick and cloying; the smell of brimstone seeped into our nostrils. It felt as though we had entered hell’s foyer.

The rail cars stopped. We stepped out and walked a short distance to a large plastic spigot protruding from the rock. A pearly stream of water trickled from the wall near the faucet’s base, forming rivulets and pools. Wafting from the water was hydrogen sulfide — the source of the chamber’s odor. Kneeling, I realized that the water was teeming with a stringy white material similar to the skin of a poached egg. Caitlin Casar, a geobiologist, explained that the white fibers were microbes in the genus Thiothrix, which join together in long filaments and store sulfur in their cells, giving them a ghostly hue. Here we were, deep within Earth’s crust — a place where, without human intervention, there would be no light and little oxygen — yet life was literally gush­ing from rock. This particular ecological hot spot had earned the nick­name Thiothrix Falls.

On a different level of the mine, we sloshed through mud and shin-high water, stepping carefully to avoid tripping on submerged rails and stray stones. Here and there, delicate white crystals, most likely gypsum or calcite, ornamented the ground and walls, glimmering like stars. We eventually reached another large spigot mired in what looked like wet clay, which varied in color from pale salmon to brick red. This, too, Casar explained, was the work of microbes — in this case a genus known as Gallionella, which thrives in iron-rich waters and excretes twisted metal spires. At Casar’s request, I filled a jug with water, scooped microbe-rich mud into plastic tubes and stored them in coolers, where they would await analysis.