Excerpt. © Reprinted by permission. All rights reserved.
Chapter 1
Intraterrestrials
Earth’s skin is full of pores, and every pore is a portal to an inner world. Some are large enough only for an insect; others could easily accommodate an elephant. Some lead only to minor caves or shallow crevices, whereas others extend into the unexplored recesses of Earth’s rocky interior. Any human attempting to journey toward the center of our planet requires a very particular type of passageway: one that is wide enough, yes, but also extremely deep, stable along its entire depth—and ideally equipped with an elevator.
One such portal sits in the middle of North America. About half a mile wide, the furrowed pit spirals 1,250 feet into the ground, exposing a marbled mosaic of young and ancient rock: gray bands of basalt, milky veins of quartz, pale columns of rhyolite, 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, South Dakota, housed the biggest, deepest, and most productive gold mine in North America. By the time it ceased operations in the early 2000s, the Homestake Mine had produced more than two million pounds of gold.
In 2006, the Barrick Gold Corporation donated the mine to the state of South Dakota, which ultimately converted it into the largest subterranean laboratory in the United States, the Sanford Underground Research Facility. After mining ceased, the tunnels began to flood. Although the lowest half of the facility remains waterlogged, it is still possible to descend nearly a mile underground. Most of the scientists who do so are physicists conducting highly sensitive experiments that must be shielded from interfering cosmic rays. While the physicists slip into bunny suits and seal themselves in polished laboratories equipped with dark matter detectors, biologists who venture into the underground labyrinth tend to seek out 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, hard hats, and, strapped to our belts, personal respirators, which would protect us from carbon monoxide in the event of a fire or explosion. The Cage descended swiftly and surprisingly smoothly, its spare frame revealing glimpses of the mine’s many levels. Our idle chatter and laughter were just audible over the din of unspooling cables and whooshing air. After a controlled plummet of about ten 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. The cars jostled forward with a sound like the rattling of heavy metal chains as the thin beams of our headlamps illuminated curving walls of dark stone threaded with quartz and specked with silver. Beneath us, I saw flashes of old railing, shallow stands of water, and rocky debris. Although I knew we were deep underground, the tunnels acted like blinders, restricting my perspective to a narrow chute of rock. Glancing at the tunnel’s ceiling, I wondered what it would feel like to see the full extent of the planet’s crust above us—a pile of rock more than three times as tall as the Empire State Building. Would our depth become palpable the way height does when you peer over the edge of a cliff? Sensing the onset of inverse vertigo, I quickly shifted my gaze straight ahead.
Within twenty 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—much closer to Earth’s geothermal heart—it was about 90°F with nearly 100 percent humidity. Heat seemed to pulse through the rock surrounding us, the air became thick and cloying, and 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. Hydrogen sulfide wafted from the water—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.”
As I gingerly probed the strands of microbes with a pen, biogeochemist Brittany Kruger opened one of several valves on the spigot before us and began conducting various tests on the discharged fluid. By simply dribbling some of the water into a blue handheld device, reminiscent of a Star Trek tricorder, Kruger measured its pH, temperature, and dissolved solids. She clamped filters with extremely tiny pores onto some of the valves to collect any microorganisms drifting through the water. Meanwhile, Casar and environmental engineer Fabrizio Sabba examined a series of rock-filled cartridges that had been hooked up to the spigot. Back at the lab, they would analyze the contents to see if any microbes had flowed into the tubes and survived within them, despite the complete darkness, the lack of nutrients, and the absence of a breathable atmosphere.
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 ornamented the ground and walls—most likely gypsum or calcite, the scientists told me. When our headlamps caught the tunnels of pitch-black rock at the right angle, the crystals shimmered like stars. After another twenty-minute journey, this time on foot, we reached another large spigot jutting out from the rock. Only half a mile underground, and better ventilated, this alcove was much cooler than the last. The rock around the faucet was 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 fracture water from the faucet, scooped microbe-rich mud into plastic tubes, and stored them in coolers, where they would await future analysis.
Kruger and Casar have visited the former Homestake Mine at least twice a year for many years. Every time they return, they encounter enigmatic microbes that have never been successfully grown in a laboratory and species that have not yet been named. Their studies are part of a collaborative effort co-led by Magdalena Osburn, a professor at Northwestern University and a prominent member of the relatively new field known as geomicrobiology.
Osburn and her colleagues have shown that, contrary to long-held assumptions, Earth’s interior is not barren. In fact, the majority of the planet’s microbes—perhaps more than 90 percent—may live deep underground. These intraterrestrial microbes tend to be quite different from their counterparts on the surface. They are ancient and slow, reproducing infrequently and possibly living for millions of years. They often acquire energy in unusual ways, breathing rock instead of oxygen. And they seem capable of weathering geological cataclysms that would annihilate most creatures. Like the many tiny organisms in the ocean and atmosphere, the unique microbes within Earth’s crust do not simply inhabit their surroundings—they transform them. Subsurface microbes carve vast caverns, concentrate minerals and precious metals, and regulate the global cycling of carbon and nutrients. Microbes may even have helped construct the continents, literally laying the groundwork for all other terrestrial life.
