Alexis templeton remembers January 12, 2014, as the day the water exploded. A sturdy Pyrex bottle, sealed tight and filled with water, burst like a balloon.
Templeton had just guided her Land Cruiser across the bumpy, rock-strewn floor of Wadi Lawayni, a broad, arid valley that cuts through the mountains of Oman. She parked beside a concrete platform that rose from the ground, marking a recently drilled water well. Templeton uncapped the well and lowered a bottle into its murky depths, hoping to collect a sample of water from 850 feet below the surface.
Wadi Lawayni is enclosed by pinnacles of chocolate-brown rock, hard as ceramic yet rounded and sagging like ancient mud-brick ruins. This fragment of the Earth’s interior, roughly the size of West Virginia, was thrust to the surface through an accident of plate tectonics millions of years ago. These exotic rocks—an anomaly on Earth—had lured Templeton to Oman.
Shortly after she hoisted her sample from the well, the bottle ruptured from internal pressure. The water gushed out through the cracks, fizzing like soda. The gas erupting from it was not carbon dioxide, as it is in soft drinks, but hydrogen—a flammable gas.
Templeton is a geobiologist at the University of Colorado at Boulder, and to her, the gas has special significance: “Organisms love hydrogen,” she says. They love to eat it, that is. The hydrogen in the sample was not, itself, evidence of life. But it suggested that the rocks beneath the surface might be the sort of place where life can flourish.
Templeton is one of a growing number of scientists who believe that the Earth’s deep subsurface is brimming with life. By some estimates, this unexplored biosphere may contain anywhere from a tenth to one-half of all living matter on Earth.
Scientists have found microbes living in granite rocks 6,000 feet underground in the Rocky Mountains, and in seafloor sediment buried since the age of the dinosaurs. They have even found tiny animals—worms, shrimp-like arthropods, whiskered rotifers—among the gold deposits of South Africa, 11,000 feet below the surface.
We humans tend to see the world as a solid rock coated with a thin layer of life. But to scientists like Templeton, the planet looks more like a wheel of cheese, one whose thick, leathery rind is perpetually gnawed and fermented by the microbes that inhabit its innards. Those creatures draw nourishment from sources that sound not only inedible, but also intangible: the atomic decay of radioactive elements, the pressure-cooking of rocks as they sink and melt into the Earth’s deep interior—and perhaps even earthquakes.
Templeton had come to Oman in search of a hidden oasis of life. That fizz of hydrogen gas in 2014 was a strong sign that she was onto something. So this past January, she and her colleagues returned, intent on drilling 1,300 feet into these rocks and finding out what lived there.
On a hot winter afternoon, a guttural roar reverberated across the sun-drenched expanse of Wadi Lawayni. A bulldozer sat near the center of the valley. Mounted on its front was a towering drill shaft, spinning several times per second.
Half a dozen men in hard hats, most of them Indian workers employed by a local company, operated the drill. Templeton and a half-dozen other scientists and graduate students congregated a few yards away, beneath the shade of a canopy that billowed in the gentle breeze. They bent over tables, examining the sections of stone core being brought up by the workers every hour or so.
The rig had been running for a day, and the cores coming out of the ground were changing color as the drill penetrated deeper into the earth. The top few feet of stone were tinted orange and yellow, indicating that oxygen from the surface had turned the iron in the rock into rusty minerals. By 60 feet below the surface, those fingerprints of oxygen petered out, and the stone darkened to greenish-gray, spider-webbed with black veins.
“This is beautiful rock,” said Templeton, running a latex-gloved finger over its surface. Her sunglasses were pushed back over her straight brown hair, revealing cheekbones darkened from years of working outside on ships, on tropical islands, in the high Arctic, and everywhere else her work has taken her. “I’m hoping we see a lot more of this,” she said.
The green-black rock was giving her a close look at something that is all but impossible to observe just about anywhere else on the planet.
These rocks from deep inside the Earth are rich in iron—iron in the form of minerals that don’t ordinarily survive anywhere near the planet’s surface. This subterranean iron is so chemically reactive, so eager to combine with oxygen, that when it comes in contact with water underground, it rips the water molecules apart. It yanks out the oxygen—the “O” in H2O—and leaves behind H2, or hydrogen gas.
Geologists call this process “serpentinization,” for the sinuous veins of black, green, and white minerals that it leaves behind. Serpentinization usually happens only in places inaccessible to humans, such as thousands of feet beneath the floor of the Atlantic Ocean.
Here in Oman, though, deep-earth rocks have been lifted so close to the surface that serpentinization occurs only a few hundred feet underground. The hydrogen gas that burst Templeton’s water bottle in 2014 was a tiny sample of serpentinization’s yield; one water well, drilled a few years ago in this same region, released so much hydrogen that it was judged an explosion risk—prompting the government to seal it shut with concrete.
Hydrogen is special stuff. It was one of the fuels that propelled the Apollo missions, and the space shuttles, into orbit; ounce for ounce, it is one of the most energy-dense naturally occurring compounds on Earth. This makes it an important food for microbes below Earth’s surface.
All told, the microbes living beneath the mountains of eastern Oman may consume thousands of tons of hydrogen each year—resulting in a slow, controlled combustion of the gas, precisely choreographed by the enzymes inside their water-filled cells.
But that hydrogen supplies only half the equation of life: To produce energy from hydrogen, microbes need something to burn it with, just as humans inhale oxygen to burn food. Figuring out what the microbes are “breathing” so far underground, beyond the reach of oxygen, is a key part of Templeton’s mission.
At two in the afternoon, a battered pickup truck trundled past the drill site on a dusty dirt track. Behind it, six camels trotted in tight formation, their heads bobbing in the air: local livestock, tethered on short leashes, being led to a fresh patch of rangeland somewhere up the wadi.
Templeton, oblivious to the camels, called out in excitement: “Gold!” She pointed to a section of core lying on the table, and to a dime-sized cluster of yellow metallic crystals. Their cubic shapes revealed her little joke: The crystals were not real gold, but fool’s gold, also known as pyrite.
Pyrite, composed of iron and sulfur, is one of dozens of minerals known to be “biogenic”: Its formation is sometimes triggered by microbes. The crystals coalesce from the waste products that microbial cells exhale. So these pyrite crystals could be a byproduct of microbe metabolism—a possibility Templeton calls “beautiful.”
Back home in Colorado, she’ll give these crystals the same careful attention that an archaeologist would devote to a Roman trash pile. She’ll cut them into transparent slices and view them under a microscope. If the pyrite is, in fact, the product of living cells, she says, then the microbes “might even be entombed in the minerals.” She hopes to find their fossilized bodies.
Not until the early 1990s did anyone suspect that abundant life might inhabit the deep earth. The first evidence came from the rocks that sit below the seafloor.
Geologists had long noticed that volcanic glass, found in dark, basaltic rocks that lay hundreds to thousands of feet below the seafloor, was often riddled with microscopic pits and tunnels. “We had no idea that this might be biological,” says Hubert Staudigel, a volcanologist at the Scripps Institution of Oceanography in La Jolla, California.
In 1992, a young scientist named Ingunn Thorseth, of the University of Bergen in Norway, suggested that the pits were the geologic equivalent of tooth cavities: Microbes had etched them into the volcanic glass as they consumed atoms of iron. In fact, Thorseth found what appeared to be dead cells inside the cavities—in rock samples collected from 3,000 feet beneath the seafloor.
When these discoveries unfolded, Templeton had not yet entered the field. She finished a master’s degree in geochemistry in 1996, then took a job at the Lawrence Berkeley National Laboratory in California, where she studied how quickly microbes were eating the jet fuel embedded in the soil of a former U.S. Navy base. A few years later, for her Ph.D. research at Stanford, she studied how underground microbes metabolize lead, arsenic, and other pollutants.
In 2002, she moved to Scripps to work with Bradley Tebo, a biology professor, and Staudigel, on a related question: How were microbes living off the iron and other metals in basaltic glass from the seafloor?
In November of that year, on the back deck of a research ship in the middle of the Pacific Ocean, she climbed down the hatch of the Pisces-IV, a car-sized submersible, and was lowered into the sea. Terry Kerby, a pilot with the Hawaii Undersea Research Laboratory, guided the craft to the southern slope of Loihi Seamount, an undersea volcano near Hawaii’s Big Island.
At a depth of 5,600 feet, the sub’s floodlights dimly illuminated a bizarre undersea landscape: a jumble of what resembled black, bulging trash bags, haphazardly stacked into towering pinnacles. These so-called pillow basalts had formed decades or centuries before as lava oozed from cracks, encountered seawater, and flash-cooled into lobes of glassy rock. Templeton lay on her side on a bench, bundled up against the cold, and watched through a thick glass portal as Kerby broke off pieces of basalt with the craft’s robotic pincer arms. Eight hours after they were lowered into the ocean, they returned to the surface with 10 pounds of rock.
The same year, she and Staudigel visited Hawaii’s Kilauea volcano, hoping to collect microbe-free volcanic glass that they could compare with their deep-sea samples. Clad in heavy boots, they walked onto an active lava flow, treading on a black crust of hardened rock just half an inch thick. Staudigel found a spot where the orange, molten lava had broken through the overlying crust. He scooped up the glowing material with a metal pole and plopped it—like hot, gooey honey—into a bucket of water. It hissed and crackled, boiling the water as it hardened into fresh glass.
Back in the lab, Templeton isolated dozens of the bacterial strains that leach iron and manganese out of the deep-sea rocks. She and her colleagues remelted the sterile glass from Kilauea in a furnace, doped it with different amounts of iron and other nutrients, and grew the bacterial strains from the seafloor on it. She used sophisticated X-ray techniques to watch, fascinated, as the bacteria digested the minerals.
“I have a basement full of basalt from the seafloor because I can’t let it go,” she told me one day during a break in the drilling.
But those rocks, and the critters that chew on them, had one major drawback for Templeton: They came from the seafloor, where the water contains oxygen.
Oxygen sustains every animal on Earth, from aardvarks to earthworms to jellyfish; our atmosphere and most of our ocean is chock-full of it. But Earth has only been highly oxygenated for a tiny fraction of its history. Even today, vast swaths of our planet’s biosphere have never encountered oxygen. Go more than a few feet into bedrock, and it’s virtually nonexistent. Go anywhere else in the solar system, including places like Mars that might harbor life, and you won’t find it, either.
As Templeton explored Earth’s deep biosphere, she had become interested in how life originated on Earth—and where else it might exist in the solar system. The subsurface could provide a window into those distant places and times, but only if she could delve deeper, below the reach of oxygen.
The mountains of east Oman seemed like the perfect place. This massive slab of slowly serpentinizing rock preserves, in its interior, the oxygen-deprived conditions and chemically reactive iron minerals that are thought to exist deep inside the planet.
Templeton and several other deep-biosphere researchers connected with a major effort that was in early planning stages—the Oman Drilling Project.
The effort was co-led by Peter Kelemen, a geologist at the Lamont-Doherty Earth Observatory in New York. He had his own mission: The deep-earth rocks in Oman react not only with oxygen and water but also with carbon dioxide, pulling the gas out of the atmosphere and locking it into carbonate minerals—a process that, if understood, could help humanity offset some of its carbon emissions.
Kelemen was present during the drilling at Wadi Lawayni in January 2018. And he was bullish on the prospects of finding life. These rocks had originally formed at a temperature of more than 1,800 degrees Fahrenheit. But they would have rapidly cooled, and today the top thousand feet of rock hover around 90 degrees Fahrenheit. These rocks, he said, “have not been hot enough to kill microbes since the Cretaceous”—the age of the dinosaurs.
At three in the afternoon at the drill site, half a dozen team members gathered near the rig for what had become an hourly ritual: a moment of suspense.
A new section of core, freshly raised from the borehole, was lowered onto a sawhorse—a stone cylinder 10 feet long and as big around as the fat end of a baseball bat, concealed in a metal pipe.
Workers lifted one end of the pipe. And out slid the core—along with a gush of black gunk. Glops of thick, dark sludge dripped on the ground. The core was covered from end to end.
“Oh my god,” someone said. “Oya.” Murmurs all around.
A worker wiped down the core, and pinprick bubbles erupted on its smooth, sheeny surface—reminiscent of the bubbles in hot cooking oil. The stone, no longer pressurized underground, was degassing before our eyes, the bubbles squirting out through pores in the rock. The odor of sewer and burnt rubber rose into the air—a smell that had instant meaning for the scientists present.
“That rock is seriously alive,” said Templeton.
“Hydrogen sulfide,” said Kelemen.
Hydrogen sulfide—a gas found in sewers, in your intestines, and, apparently, underground in Oman—is produced by microbes living in the absence of oxygen. Deprived of that life-giving gas, they pull a trick that no animal on Earth can do: They breathe something else. In other words, they burn their food using some other chemical that is available underground.
The sections of core brought up so far offered clues about what they might be breathing. The gassy core was crisscrossed by bands of orange-brown stone—marking the places where hot magma had spurted through deep fissures in the Earth millions of years before, when this rock lay miles underground.
Those bands of fossil magma would have gradually bled their chemical components into the groundwater—including a molecule called sulfate, which consists of a single sulfur atom studded with four oxygen atoms. The microbes were probably using this molecule to digest hydrogen, said Templeton: “They eat the hydrogen and they breathe the sulfate.” And then, they exhale fart gas.
Hydrogen sulfide isn’t just stinky. It is also toxic. So the very microbes that produce it also run the risk of poisoning themselves as it accumulates underground. How did they avoid doing so? Once again, the rock provided clues.
As drilling continued over the next several days, the black goo petered out. Each new section of core was dry and stink-free. But the stone itself had changed: Its mosaic of veins and serpentine minerals had darkened into shades of gray and black, like a plaid shirt soaked in ink.
“All of that blackening is a bio-product,” Templeton said one afternoon, as she and her research associate, Eric Ellison, crowded inside a cramped laboratory trailer, packing samples of rock to send home. Some of the rocks sat in a sealed Plexiglass box, and Ellison handled them with his hands inserted through gloves mounted in the walls of the box—giving the appearance that the rocks contained something sinister. But the precaution wasn’t intended to protect humans; it was meant to keep the delicate microbes out of contact with oxygen.
Templeton speculated that the microbes had stained the most recent rock samples: The hydrogen sulfide they exhaled had reacted with iron in the surrounding stone, creating iron sulfide—a harmless black mineral. The pyrite minerals we’d seen earlier were also composed of iron and sulfide, and could have formed the same way.
These black minerals are more than an academic curiosity. They provide a glimpse of how microbes have not only survived inside the Earth’s crust, but also transformed it, in some cases forming minerals that might not otherwise exist.
Some of the world’s richest deposits of iron, lead, zinc, copper, silver, and other metals formed when hydrogen sulfide latched onto metals that had dissolved deep underground. The sulfide locked the metals in place, concentrating them into minerals that accumulated for millions of years—until they were exhumed by miners. The hydrogen sulfide that formed those ores often came from volcanic sources, but in some cases, it came from microbes.
Robert Hazen, a mineralogist and astrobiologist at the Carnegie Institution in Washington, D.C., believes that more than half of Earth’s minerals owe their existence to life—to the roots of plants, to corals and diatoms, and even to subsurface microbes. He has even speculated that the world’s seven continents may owe their existence, in part, to microbes gnawing on rocks.
Four billion years ago, Earth had no permanent land—just a few volcanic peaks jutting above the ocean. But microbes on the seafloor may have helped change that. They attacked iron-rich basalt rocks, much as they do today, converting the volcanic glass into clay minerals. Those clays melted more readily than other rocks. And once melted, they resolidified into a new kind of rock, a material lighter and fluffier than the rest of the planet: granite.
Those buoyant granites piled into heaps that rose above the ocean, creating the first permanent continents. This would have happened to some degree without the help of microbes, but Hazen suspects that they accelerated the process. “You can imagine microbes shifting the balance,” he says. “What we’re arguing is that microbes played a fundamental role.”
The emergence of land had a profound effect on Earth’s evolution. Rocks exposed to the air broke down more quickly, releasing trace nutrients such as molybdenum, iron, and phosphorus into the oceans. These nutrients spurred the growth of photosynthetic algae, which absorbed carbon dioxide and exhaled oxygen. Just over 2 billion years ago, the first traces of oxygen appeared in Earth’s atmosphere. Five hundred and fifty million years ago, oxygen levels finally rose high enough to support the first primitive animals.
Earth’s abundant water, and its optimal distance from the sun, made it a promising incubator for life. But its evolution into a paradise for intelligent, oxygen-breathing animals was never guaranteed. Microbes may have pushed our planet over an invisible tipping point—and toward the formation of continents, oxygen, and life as we know it.
Even today, microbes continue to make, and remake, our planet from the inside out.
In some ways, the microbe underworld resembles human civilization, with microbial “cities” built at the crossroads of commerce. In Oman, the thriving oasis of stinky, black microbes sat 100 feet underground, near the intersection of several large rock fractures—channels that allowed hydrogen and sulfate to trickle in from different sources.
Elisabetta Mariani, a structural geologist from the University of Liverpool in England, spent long days under the canopy, mapping these breaks in the rock. Late one morning, she called me over to see something special: a break cutting diagonally across a core, exposing two rock faces streaked in paper-thin layers of green-and-black serpentine.
“Can you see here these grooves?” she asked, in English accented with her native Italian, pointing out scratches that raked the two serpentine faces. They showed that this was more than just a passive break; it was an active fault. “Two blocks of rocks have slipped past each other along this direction,” she said, gesturing along the grooves.
Tullis Onstott, a geologist at Princeton University not affiliated with the Oman drilling, believes that such active faults may do more than just provide routes for food to move underground—they may actually produce food. In November 2017, Onstott and his colleagues began an audacious experiment. Starting from a tunnel 8,000 feet down in the Moab Khotsong gold mine in South Africa, they bored a new hole toward a fault that lay nearly half a mile deeper still. On August 5, 2014, the fault had sparked a magnitude-5.5 earthquake. By drilling into it, Onstott hoped to test the provocative idea that earthquakes supply food to the deep biosphere.
Scientists have long noticed that hydrogen gas seeps out of major faults such as the San Andreas in California. That gas is produced in part by a chemical reaction: Silicate minerals pulverized during a quake react with water and release hydrogen as a byproduct. For microbes sitting next to the fault, that reaction could result in something like a periodic sugar rush.
In March 2018, four months after the drilling in the Moab Khotsong mine began, workers brought up a stone core that crossed the fault.
The rock along the fault was “pretty banged up,” says Onstott—torn with dozens of parallel fractures. The stone lining some of those cracks was crushed into fragile clay, marking recent earthquakes. Other cracks, filled with veins of white quartzite, marked older ruptures from thousands of years before.
Onstott is now searching those quartzite veins for fossilized cells and analyzing the rock for DNA, in hopes of finding out what kind of microbes—if any—inhabit the fault.
More importantly, he and his colleagues have kept the borehole open—monitoring water, gases, and microbes in the fault, and taking new samples each time there’s an aftershock. “You can then see whether or not there’s a gas release,” he says, “and whether or not there’s a change in the microbial community because they’re consuming the gas.”
Even as Onstott awaits those results, he is starting to consider an even more radical possibility: that deep-dwelling microbes don’t just feed off of earthquakes, but might also trigger them. He believes that as microbes attack the iron, manganese, and other elements in the minerals that line the fault, they could weaken the rock—and prime the fault for its next big slip. Exploring that possibility would mean doing laboratory experiments to find out whether microbes in a fault can actually break down minerals quickly enough to affect seismic activity. With a scientist’s characteristic understatement, he contemplates the work ahead: “It’s a reasonable hypothesis to test.”
By january 30, the drill in Wadi Lawayni had reached a depth of 200 feet. Its motor growled in the background as Templeton and her colleague, Eric Boyd, rested in camp chairs under an acacia tree. Strewn at their feet lay the signs of other travelers who had paused in this rare island of shade—nodules of camel dung, smooth and round like leathery plums.
“We think that this is an environment that’s important for understanding the origins of life,” said Boyd, a geobiologist from Montana State University in Bozeman. That potential, he said, is part of what lured him and Templeton to these deep-earth rocks in Oman: “We like hydrogen.”
Both Boyd and Templeton believe that life on Earth started in an environment similar to that which lies a few yards beneath their camp chairs. They believe that life began within subsurface fractures, where iron-rich minerals gurgled out hydrogen gas as they reacted with water.
Of all the chemical fuels that existed on Earth 4 billion years ago, hydrogen would have been one of the easiest for early, inefficient cells to metabolize. Hydrogen wasn’t only produced by serpentinization, either; it was also produced—and still is, today—by the radioactive decay of elements such as uranium, which constantly splits apart water molecules in the surrounding rock. Hydrogen is so labile, so willing to break apart, that it can even be digested using sluggish oxidants, like carbon dioxide or pure sulfur. DNA studies of millions of gene sequences suggest that the forerunner of all life on Earth—the “last universal common ancestor,” or luca—probably did use hydrogen as its food, and burned it with carbon dioxide. The same might be true for life in other worlds.
The iron minerals that exist here in Oman are common across the solar system, as is the process of serpentinization. The Reconnaissance Orbiter, a space probe now circling Mars, has mapped serpentine minerals on the Martian surface. The space probe Cassini has found chemical evidence of ongoing serpentinization deep within Saturn’s ice-covered moon, Enceladus. Serpentine-like minerals have been detected on the surface of Ceres, a dwarf planet that orbits the sun between Mars and Jupiter. Serpentine minerals are even found in meteorites, the fragments of embryonic planets that existed 4.5 billion years ago, just as Earth was being born—raising the possibility that the cradle of life’s origin actually existed before our planet did.
Hydrogen—an energy source for nascent life—was produced in all of these places. It is probably still being produced throughout the solar system.
To Boyd, the implications are breathtaking.
“If you had rock like this, at a temperature similar to Earth, and you had liquid water, how inevitable do you think life is?” he asked. “My personal belief is, it’s inevitable.”
Finding that life will be a challenge. With existing technologies, a probe sent to Mars could drill no more than a few feet below its hostile surface. Those shallow rocks might contain signs of past life—perhaps desiccated carcasses of Martian cells, sitting inside the microscopic tunnels that they chewed into the minerals—but any living microbes are likely to be buried hundreds of feet deeper. Templeton has grappled with the problem of detecting past signs of life—and of distinguishing those signs from things that happened without the influence of life—ever since she started looking at basaltic seafloor glasses 16 years ago.
“My job is to find bio-signatures,” she says. As she studies the rocks drilled out of Oman, she’ll subject them to some of the same tools that she used on the glasses. She will bounce X-rays off the mineral surface in order to map how the microbes are altering the minerals, and whether they are leaving metals in place or etching them away. By learning how living microbes chew on minerals, she hopes to find reliable ways of identifying those same chemical chew marks in extraterrestrial rocks that haven’t held living cells for thousands of years.
One day, these tools might be packed onto a Mars rover. Or they might be used on rocks that are brought back from other worlds. For now, she and her colleagues have plenty to do in Oman, figuring out what inhabits the dark, hot, hidden biosphere below their feet.
编者按：本文编译自The Atlantic题为“Meet the Endoterrestrials”的文章，作者Douglas Fox。科学家一直希望寻找生命的起源，如今他们把眼光放在了地下世界的微生物上。它们外表难以分辨，却“悄无声息”实现了人类梦寐以求的氢能利用，并为地球陆地，矿藏及化石能源的形成做出了不可磨灭的贡献，指导人类寻找外星生命的痕迹。
2014年的1月12号，Alexis Templeton记得那天一个装满水的玻璃瓶竟像气球般炸开了。
Templeton开着越野车穿越Wadi Lawayni山谷布满石砾的颠簸路面，这个广袤的山谷终年干旱，横穿阿曼的延绵群山。她把车停在地面伸出的一个混凝土平台旁，那里标识了一个新近钻出的水井。Templeton揭开井盖，吊着水瓶深入阴暗的地下，尝试取得地底下850英尺深处的水样。
这个山谷四周被深褐色的岩峰环绕，质地如陶瓷般坚硬，但滚圆和下垂的外形看起来却像古时的泥砖废墟。它们原本处于地球深处，面积和西弗吉尼亚州相近，数百万年前由于板块运动“横空出世”。正是这些特别的岩石吸引Templeton来到阿曼。
她把水样从深井中取出，不一会儿，由于气压变化，水瓶发生了爆炸，水样像碳酸饮料般飞溅，但其中逸出的气体和汽水不同，不是二氧化碳，而是可燃的氢气。
Templeton是科罗拉多大学一名地质生物学家。发现氢气对她而言意义重大。“有些生物体非常喜欢氢气，并以此为生命基础”，尽管水样中的氢气不能直接证明地下生命的存在，但它能证明地底岩石可以成为部分生命的栖身之所。
越来越多科学家相信，在地球的深处孕育着形形色色的生命，Templeton正是其中一员。据估计，这些未确认的生命大概占到全球所有生命形式的十分之一以上，甚至高达半数。
在落基山脉，科学家曾发现有微生物生活在地底下6000英尺深的花岗岩；在海床沉积物中，恐龙时代的微生物的生命痕迹也得以重见天日；在南非的金矿，地下11000英尺的深处，一些微小的动物——细长的蠕虫，虾形节肢动物，长有胡须的轮虫也得以发掘。
人们常常把地球看作是一个实心的岩球，生命只在球面周围活动。但科学家眼中，地球更像个芝士球，里面的微生物终年不断地啃咬，腐蚀着坚固厚实的外壳。与人们想象中的进食不同，这些生物不用动口，在无形中就能获取营养：放射性元素的衰变能，沉入地核高压熔化的岩石，甚至一场地震，都能成为它们获取营养的手段。
Templeton为了寻找隐藏的生命来到阿曼。2014年的那场小爆炸让她有了强烈的预感：好像找到什么了。今年1月，她们重回阿曼，深入山谷，挖掘地下深处的岩石，试着找到生命的痕迹。
阿曼的冬季依旧炎热，一个下午，Wadi Lawayni山谷阳光普照，一阵阵低沉的咆哮震彻山谷。山谷中央有一台巨大的推土机，前面耸起一根钻杆，分秒不息地转动着。
五六个当地公司雇佣的印度工人带着安全帽操纵着地钻。Templeton和另外几个科学家以及研究生聚集在几码外的凉棚下，围着桌子仔细检查勘探工人每小时拿过来的岩芯样本。
地钻整整工作了一天，随着勘探深度的增加，挖出来的岩芯颜色渐渐有了变化。起初几英尺是橙黄色的，这是因为地表的氧气把岩石中的铁转化成了锈。当深度大于60英尺后，氧气的作用逐渐消失，岩石的外表变暗泛绿，并布满黑色的条纹。
“这石头太漂亮了”，Templeton带着乳胶手套，用手指感受石块表面的起伏。这些年，她为了工作曾乘船出海，到热带岛屿，高寒的北极等无数的地方。所以当她将太阳眼镜架在自己棕色的长发之上，我们看到了她晒黑的脸颊。
这块绿得发黑的岩石是在地球的其它角落几乎不可能观察到的。这些蕴藏于地球深处的岩石富含铁元素——但与地球表面的铁有所不同。地底的铁化学性质十分活泼，易与氧气反应。当这种岩石接触到地下水，水分子发生分解，氧原子被夺走，便形成了氢气。
由于反应后，岩石表面会留下黑白或青绿的蜿蜒条纹，因此地质学家称这个过程为蛇纹石化作用。蛇纹石化通常发生在人类无法到达的地方，例如大西洋海底以下几千英尺的岩石层。
不过由于阿曼的深层岩石被板块运动抬升得非常接近地表，因此蛇纹石化只发生在地底下几百英尺以内的区域。引起2014年那次水瓶爆炸，不过是十分轻微的蛇纹石化作用。在同一地区，数年前也曾开凿过一座水井。然而过高的含氢量让其成为了危险的爆炸源，因此政府用混凝土将其彻底密封。
氢气是一种特殊的资源，能用作航天燃料，推动宇宙飞船航行。它是地球上能量密度最高的天然化合物之一，这让氢气成为地表之下微生物重要的营养来源。微生物细胞中的酶可以精确地控制氢气的消耗，让反应处于温和可控的状态。
但好比人类吸入氧气进行代谢，微生物还需要一些助燃物，才能从氢气中获取能量。找出微生物如何在贫氧的地底进行代谢的答案，就成了Templeton的关键任务。
挖掘现场，Templeton指着桌子上一小簇黄色的晶体，突然惊叫：“看，有金子！”然而它的立方结构“出卖”了她：这是一种“愚人金”，叫做黄铁矿，由铁与硫元素组成，是生命必须的矿物质之一。由于黄铁矿晶体可以在微生物排出的废物中聚集而成，Templeton认为她看到的晶体可能正是微生物代谢的副产物。
回到科罗拉多后，她小心地处理收集回来的晶体，将其切割成透明薄片，用显微镜进行观测。如果这些黄铁矿晶体是生命的产物，那么“肯定有微生物聚居于此”，她希望能发现微生物的化石结构。
“海底二万里”
长久以来地质学家注意到，海床下数百公尺深处的黑色玄武岩上布满了火山玻璃，在显微镜观测下，往往能发现奇怪的蚀坑与孔道。“我们从未想过这是生物的作用。”来自加州斯克里普斯海洋研究所的火山学家Hubert Staudigel如此说道。
1992年，来自挪威卑尔根大学的一名年轻科学家Ingunn Thorseth，提出猜想，坑洞在地质上类似蛀牙：微生物以玻璃中的铁元素为生，因此造成了一定的腐蚀。Thorseth还发现了有力的证据：在海床下3000英尺深处的岩石标本中，他们发现了在坑洞中凋亡的微生物。
该研究公开时，Templeton还未进入这个领域。她在1996年获得了地质化学的硕士学位，随后在加州的劳伦斯伯克利国家实验室供职，在那里她发现，一些微生物能够快速消耗残留在前海军基地土壤的飞机燃料，以维持生命活动。数年后，她在斯坦福大学进行了博士阶段的学习，研究了地底微生物对铅，砷等污染物的代谢机制。
2002年，她来到斯克里普斯，与生物学家Bradley Tebo和Staudigel共事。他们试图搞清楚一个问题：海床玄武岩的铁及其它金属是如何维持微生物的生命？
同年11月， Templeton从乘潜水艇潜入深海，向罗希海底山的南坡进发，这是一座位于夏威夷大岛附近的海底火山。下潜至5600英尺处，潜水艇开启大灯照亮了一幅海底奇观：一座黧黑的海底石林，突兀地堆叠成无数的尖峰。火山岩浆从裂缝流出，遇到冰冷的海水后，迅速冷却成玻璃岩碎屑，这些所谓的枕状玄武岩，形成的时间已经超过数十甚至上百年。Templeton侧身躺在长凳上，蜷缩着身子以抵御寒冷，透过玻璃窗看着机器手臂掰下小块的玄武岩。下潜8小时后，他们带着10磅重的岩石样本回到了海面。
她曾告诉记者：“我家地下室里摆满了从海床收集来的玄武岩块，我一块也不舍得扔掉。”
在研究地球深处的生物圈时，Templeton对生命的起源产生了兴趣，并开始思考太阳系还有哪些地方会有生命的存在。对地下世界的探索好比推开了宇宙研究的一扇窗，但前提是她的挖掘计划要深入无氧的地下区域。
纵观历史，过去很长的一段时间内，地球的氧含量并不高。即使在今天，地球生物圈还有很大的区域处于无氧环境中。海床下方不远处，氧气就几乎不存在了，放眼太阳系各大星球，即使像火星这般可能拥有生命的星球，大气中也没有氧的存在。
阿曼东部山区是非常理想的研究地点。这里保存着大量缓慢蛇纹石化的岩石样本，岩石内部富含化学性质活泼的铁元素，这些都是原本地下深层岩石才有的特质。为此，Templeton和其他几名研究者共同努力，推动了阿曼钻井项目的早期开展。
这项研究由纽约天文台的地质学家Peter Kelemen牵头。他本人也有自己的研究方向：他发现这里的岩石不光能与水和氧反应，还能使二氧化碳从大气中分离，收集在矿物质内部。这项反应机理的研究将会为碳减排提供新的解决方案。
Kelemen参与了今年1月的钻井研究。他对能否找到生命痕迹表现得很乐观。尽管这些岩石是在高达1000多度的环境中形成的，但冷却过程非常迅速，且目前周围的环境温度不过30多摄氏度。他认为，“这样的条件下，自恐龙时代的微生物也是可以得到保留的。”
硫——微生物代谢的奥秘
下午三点，科考队员们聚集到钻井平台旁，等待着每个小时的例行工作，这是一个充满悬念的时刻。
刚从深井中取出的新石块，呈圆筒状，长10英尺，厚度和棒球棍相仿，被放在锯架上，再密封到金属管中。
随后工人们举起了管子的一端，随着一股黑色的粘稠物喷涌而出，岩心滑了出来。厚厚的黑色污泥滴在地上，岩芯表面黑漆漆的。
研究人员将岩芯擦拭干净，看到细小的气泡从光亮细滑的岩石表面逸出——这让人想起滚油里的泡沫。由于没有了地底的高压，石头在人们眼前会迅速释放储存的气体，一个个气泡从孔道中喷薄而出，蔓延着一股奇异的气味，既像是烧焦的橡胶，又有点像是阴暗的下水道。科学家们好像想到了气体的来源。
“里面有硫化氢。”
硫化氢是一种通常出现在下水道，或者人体肠道内的气体。硫化氢的形成要归功于无氧呼吸的微生物。尽管缺少了氧气，它们找到了另类的生存方式：用地底其它化学物质进行呼吸作用获取营养。
这段嘶嘶作响的岩芯让科学家注意到，微生物活动需要的到底是什么物质。岩芯由橙黄色的石头交错而成，说明数百万年前岩浆在石块的附近喷涌流动。
岩浆化石逐渐将里面的化学成分释放到地下水当中——其中包括硫酸根离子。Templeton说：“微生物可能正是依靠吸入硫酸根离子消耗氢气，再排除废气。”
硫化氢除了气味呛人，还有强烈的毒性。由于硫化氢的不断累积，那些排出硫化氢的微生物实际上还冒着自己中毒的风险。它们怎么避免自身受到伤害？科学家再一次从石头中找到了线索。
地钻继续工作数天后，黑色的淤泥逐渐消失。每一段岩芯都是干净而无异味的。但石头本身早已发生了变化，原有的纹路和蛇纹石矿物变得灰黑一片。“变黑的部分都代表着生化反应的产物。”Templeton认为微生物呼出的硫化氢和岩石的铁元素发生反应，生成了黑色的硫化亚铁。先前看到的黄铁矿也是由铁与硫组成，形成的方式大致相同。
硫化亚铁的存在让研究人员了解微生物是如何生活和改造地底世界，甚至合成出原本不存在的矿物质。硫化氢与在地下深处溶解的金属相结合，形成了地球上众多重要的金属矿藏，包括铁、铅、锌、铜、银矿等等。硫对金属元素的固定与富集作用，使其得以在漫长的岁月中积累——直至被开采出来。
来自华盛顿特区卡内基研究所的矿物学与天体生物学家Robert Hazen，相信地球上超过一半的矿物对生命活动有着重要的作用——包括植物，珊瑚和硅藻，地下世界的微生物群落等等。他甚至推测大陆的形成与微生物的侵蚀密不可分。
40亿年前，地球上还没有稳定存在的大陆，只有一些火山跃出海平面。后来海床的微生物栖息在富铁的玄武岩，将火山玻璃转化为黏土矿物。黏土矿物不如玄武岩坚硬，重新固化后形成了一种轻质而蓬松的材料——花岗岩。
陆地的出现对地球的进化有着深远的影响。暴露在大气中的岩石更易风化，释放痕量的化学物质，如钼，铁，磷元素。化学元素汇入海洋促进了藻类的繁殖。藻类能吸收二氧化碳，并呼出氧气。到了20亿年前，大气中开始出现痕量的氧气，而在5.5亿年前，氧气含量已经足够维持早期原始动物的生命活动。
地球丰富的水资源与合适的日地距离，让它成为最理想的生命摇篮。但当时的条件还不足以让地球成为智人生活的理想环境。是微生物的存在让我们的星球朝着形成陆地、氧气和高级生命的趋势发展。时至今日，微生物依旧在由内而外地重塑着这个世界。
放眼太空
某种程度上，地下世界的微生物好比人类文明的发展，微生物的“城市”往往修建在繁忙的“交通要道”旁。上文提到的微生物群落，生活在地下100英尺的深处，附近是几处大岩石裂缝的交汇点，正好允许氢气与硫酸盐从四面八方流向群落。
来自英国利物浦大学的结构地质学家Elisabetta Mariani，花了很长时间才描画出了岩石裂缝的图像。一天清晨，她向记者展示了一幅特别的图画：一条裂缝从岩石对角线穿过，露出了浅薄而绿黑相间的蛇纹石表面。
“你能看到表面的凹槽吗？”她指着破开的岩石表面内部的划痕问记者。他们认为这并不是环境造成的痕迹，她用手比划着，“这是两块石头相互摩擦形成的”。
普林斯顿大学的地质学家Tullis Onstott认为，断层除了引导气体通向微生物群落以外，还可能具有生产营养物质的作用。2017年11月，他和同事在南非的Moab Khotsong金矿底下8000英尺处，朝断层钻出了一个半尺深的洞。三年前的8月5日，该断层曾引起一场5.5级的地震，Onstott希望这次挖掘能发现地震与地底生物圈营养物质来源的联系。
长久以来研究人员注意到氢气会从主要断层逸出，例如加州的圣安地列斯。氢气的产生源自一种化学反应：硅酸盐矿物在震动中变成粉末，与地下水反应会生成氢气。
2018年3月，Moab Khotsong金矿开掘四个月后，工人们拿到了断层区域的岩芯样本。
“断层附近的岩石有严重撞击的痕迹，”Onstott说——被撕裂成数十条平行裂缝。其中一些裂缝里，石头被压成易碎的粘土，这是上次的地震所造成的。其余的裂缝，被白色石英岩脉填充，代表了数千年前的地质断裂。
Onstott在石英岩脉中寻找化石细胞，并分析岩石中的DNA，希望能找到在断层中栖息的微生物。Onstott和同事一直保持洞口敞开，监控断层内部的水气含量与微生物活动，并在每次余震后收集岩石样本。他表示：“我们可以了解石头是否在放气，以及了解微生物消耗气体的过程中，群落是否有所变化。”
就在Onstott等待结果的时候，他开始考虑一种更激进的可能性：微生物不光是依靠地震获取营养，甚至能引起一场地震自给自足。微生物消耗断层矿物中的铁、锰等其它元素，这一过程会破坏岩石的结构，引起下一次断裂。要想验证这一点，我们必须测定断层内微生物对化学物质的消耗速度是否足以影响地震活动。带着科学家独有的镇定，他认真思考，深信“这是一个合理的假设。”
今年1月30日，Wadi Lawayni山谷的地钻已经下挖至200英尺，研究人员每天听着发动机的轰鸣声工作，休息。 “这里会是认识生命起源的一个重要场所。”来自蒙大拿州的地质生物学家Boyd这样说道。对未知的探索吸引了他们这些科学家驻扎在此。“每次发现氢气都使我们无比兴奋。”
在40亿年前就已存在的众多化学燃料当中，早期生命最容易进行代谢的莫过于氢气。氢气的生成不光来自蛇纹石化反应，还有目前广泛利用的放射性衰变反应。氢气性质活泼，遇到如二氧化碳和硫粉等弱氧化剂也能发生解离。大量的基因序列测试表明，地球生命的共同祖先大概都利用氢气与二氧化碳的反应获取能量，这同样适用于其它星球可能存在的生命形式。
许多勘测轨道飞行器及太空船，都能拍摄到地球外许多行星表面存在蛇纹石化矿物，或是找到反应持续进行的化学证据。甚至在陨石的表面也监测到了蛇纹石状的矿物。这说明在45亿年前，地球刚刚诞生之时，胚胎行星的碎片就已经存在，暗示着生命起源很可能早于地球的出现。
“一块类似的石头，放在和地球温度相仿的地方，浇上液态水，你不可避免地会和我们想到一块。”
但对太空的探索依然是个挑战。现有的科技仅允许我们从火星上钻取几英尺深的岩石样本，上面或许保留着在火星早已逝去的生命痕迹——但真正存活的微生物组织应该会生活在更深的岩层中。Templeton正在着手解决检测过去生命痕迹的难题，以及辨别痕迹是否由生命活动所造成。这项课题从她开始研究海底玄武岩就已开始，至今已过去16年。
“我的目标是要找到生命的标志，”她会将勘探出的岩石样本送检，用X射线击打矿物表面，获取微生物与矿物分布的图谱，了解微生物与矿物间存在的行为。她希望通过研究微生物腐蚀矿物的方式，找到辨识外太空岩石样本的生物蚀刻标记的手段，而这些岩石，可能已有上千年未被生命垂怜。
我们畅想未来某天，这些工具会送到火星勘探车上，或者用在从其它星球采回的岩石标本。至于现在，Templeton还在阿曼寻找着，阴暗炎热的地底世界里所潜藏着的秘密。
