I awoke face-down in the dirt, drenched with sweat, engulfed in darkness so absolute that it made no difference whether my eyes were open or closed. Groggy and disoriented, I sat bolt upright and tried to figure out where I was. Then I remembered, and had to fight back a wave of panic: I was a thousand feet underground, in the midst of a claustrophobic labyrinth called Lechuguilla Cave.

I groped inside my sleeping bag until I found a flashlight, and flicked it on. The beam illuminated a low, domed space the size of a parking garage festooned with preposterous limestone udders. Nine other people were sprawled on the ground nearby. Three of them—Chris McKay, Penny Boston, and Larry Lemke—were NASA scientists who had descended into this disquieting netherworld because, as McKay had explained earlier with an apparent non sequitur, “We want to know if there’s life on Mars.”

It was not easy getting here. The mission had been launched two days earlier, from a scorched New Mexico hillside freckled with prickly pear and lechuguilla plants—the spiny agave after which the cave was named. Located in Carlsbad National Park, just a few miles from Carlsbad Caverns, the unmarked entrance to Lechuguilla is a forbidding vertical shaft. Wearing helmets, headlamps, mountaineering harnesses, and 50-pound backpacks, we attached ourselves to a frayed rope, shuffled backwards over the edge, and rappelled one by one into the gloom. Within moments we found ourselves in an environment that felt more alien than any place I’d ever been.

Presently considered to be one of the most spectacular and geologically unique caves in the world, until nine years ago Lechuguilla was known to extend no further than its 90-foot entrance pit. At one edge of the pit, however, recreational cavers noticed a distinct breeze blowing from a pile of jumbled rocks, inspiring them to probe deeper. On May 25, 1986, after a decade of intermittent excavation, three men dug through the last of the rubble and discovered a narrow, twisting passageway leading down into the bowels of the earth.

One of the trio who made this discovery was a celebrated caver named Rick Bridges, and it was he who led the NASA team into Lechuguilla last December. The trip down was long and arduous. The temperature remained a constant 67 degrees F., which initially seemed quite comfortable. The humidity hovered near 100%, however, so the slightest physical effort produced a flood of perspiration that soaked our clothing and never dried.

Bearing all our food and equipment on our backs, we scrambled through a chaos of truck-size boulders, slithered like lizards down narrow slots, and dangled above chasms that appeared bottomless. At times we used ropes and technical caving gear to rappel into pits that overhung on all sides. Knowing that the only way to return to the surface five days hence would be to climb strands of rope we left behind as we descended, my anxiety grew as we pressed deeper and deeper underground.

Our battery-powered headlamps became our most valuable possessions. Indeed, each of us carried at least two back-up lights, because without a means of illumination we would be blind, helpless, and stranded. Thus far more than eighty miles of passageway have been discovered in Lechuguilla, a natural maze of mind-boggling complexity that looks like a tangle of spaghetti when plotted on a three-dimensional map. Bridges guided us down a route that corkscrewed through a devious geologic honeycomb.

Although fraught with hazards, Lechuguilla is a place of phantasmal beauty. Cramped, twisting passages opened suddenly into spaces as voluminous as Madison Square Garden, the walls of which sparkled with brilliant white crystals. Opalescent “cave pearls” lay in clusters at the bottom of shallow pools. Fragile balloons of hydromagnesite, formed by expanding gases, decorated the ceiling of a low tunnel. In several places delicate gypsum “chandeliers” hung from the roof like flocked Christmas trees. Everywhere I turned there was one or another exotic embellishment, otherworldly and dazzling.

The NASA crew, however, was uninterested in such comely sights. What had lured them into Lechuguilla, rather, were the ugliest parts of the cave: a handful of comparatively unadorned areas coated with a nasty, mud-like substance known as corrosion residue. Cavers called it “gorilla shit,” and it was impossible to touch the stuff without becoming filthy, but McKay, Boston, and Lemke found it extremely stimulating. They believed that corrosion residue might hold important clues about Mars—an object of passionate interest to all three scientists, and their sole reason for visiting Lechuguilla. Specifically, they theorized that corrosion residue might answer crucial questions about whether life exists on the red planet.

“You cavers,” McKay explained to Bridges, “have a pathological defect that allows you to think it’s fun to spend five days underground, getting dirty, lugging 50-pound packs up and down cliffs. You have my sympathy, and I hope some day they find a cure for your disease, but for Penny and Larry and me this is not fun. I’d much rather be home right now. But the corrosion residue is so exciting—and what it might tell us about Mars is so exciting—that we’re willing to come down into this cave to suffer miserably and grunt like pigs in order to study it.”

Photographs of Mars show that water once flowed there, and long ago it may also have had a fairly substantial atmosphere. Today the planet is a harsh place, however, brutally cold and without liquid water. The atmosphere is exceedingly thin. “The surface would seem to be very inhospitable to life of any kind,” McKay acknowledges. “If life does exist on Mars, you’d expect to find it underground. Organisms would be shielded from the intense ultraviolet radiation there. A volcanic heat source might conceivably generate liquid water from subsurface ice. In the absence of sunlight and organic matter, any extant life would have to derive its energy entirely from mineral sources.” To put it simply, such life would have to eat rocks.

Creatures of that ilk do in fact exist here on earth. Biologists call them autotrophic organisms, and they have been found living around hot volcanic vents on the ocean floor, among other places. Not a lot is known about them, but McKay, Boston, and Lemke speculate that similar forms of life might flourish in caves on Mars—and in the dark recesses of Lechuguilla Cave.

***

Deep in Lechuguilla, many of the fundamental reference points that lend order to everyday existence are absent. There is no weather in the cave, no natural horizon, no noise, no sunrise or sunset, no way to tell what time it is, what day, what year. Lacking visual cues, we find ourselves keeping odd hours. Crawling out of his bivouac sack at 10 AM, Larry Lemke—a compact, 47-year-old engineer from NASA’s Ames Research Center—declares, “My biological clock is already drifting. I seem to fall into a 25-hour day down here—just like you would on Mars, which has 24-and-a-half-hour days.”

Stiff and hung-over from the strenuous trip down, the team assembles for breakfast on a gravelly hummock. This chamber, named Deep Secrets by the cavers who discovered it, will serve as our basecamp for four nights. I dig into a plastic pouch of cold freeze-dried chili leftover from yesterday’s dinner—not because it tastes good, but because if I don’t eat it I’ll have to carry it back up to the surface. To keep Lechuguilla pristine, everything that came in with us must go out with us. Even human waste is sealed carefully in Ziploc bags and carried out.

We are on our own down here, cut off from the rest of the world. The implications of that are not lost on anyone. In 1991 an experienced caver named Emily Mobley broke her femur not far from the Deep Secrets camp when an immense rock rolled onto her leg. It took the combined efforts of 180 cavers four days to evacuate her.

Our isolation—and our need to be self-sufficient—is analogous to a space journey, suggests Lemke, who has designed several hypothetical missions to Mars for NASA. “This cave is an alien world,” he says, “with its own special rules and hazards. Living down here is difficult. It’s given me some practical insights into the challenge of doing research half-way across the solar system.”

The research scheduled for this morning’s agenda will be conducted a mere quarter-mile from camp, but getting to the site is problematic enough to keep plenty of insights flowing Lemke’s way. After loading up our backpacks with scientific paraphernalia, we negotiate a passage called the Fortress of Chaos, which is rather like squirming through an enormous chunk of Swiss cheese. At the top of the Fortress a strand of rope hangs from a hole overhead. Clipping small, ratchet-like devices to the rope (called ascenders, they slide upward freely, but grip the line when weighted), we haul ourselves toward the ceiling with much huffing and puffing.

The rope leads through a sugary white flue of gypsum crystals to emerge in a chamber stacked with huge, tottering boulders. When they see that the entire room is coated with a filthy brown scum of corrosion residue, McKay, Boston, and Lemke grow ecstatic.

Chris McKay, 40, is a 6’6” beanpole of a man with a scraggly beard and a wry sense of humor. 41-year-old Penny Boston is smallish and voluble. A tangle of blond curls cascades haphazardly from beneath her helmet. Neither McKay nor Boston is particularly athletic, or could be called a thrill seeker. Each left behind a spouse and a young child to make the trip into Lechuguilla.

Given that both McKay and Boston claim to abhor being underground, one has to wonder, Why didn’t they simply ask a team of experienced cavers to collect a sample of corrosion residue and deliver it to their labs, thereby obviating the need for either scientist to suffer the claustrophobic hardships of Lechuguilla?

Such a question, McKay observes, “gets to the very nature of field work. Why couldn’t we simply use surrogates? I don’t really know the answer, but I know it doesn’t work. There is just no substitute for being there in person. Experiencing the environment with your own eyes and your full attention—both conscious and unconscious—is essential. Surrogates can’t give us the understanding we develop by going into the field ourselves, taking samples in real time, and discussing things on site.”

Hence the numerous extended visits McKay has made over the past fifteen years to places such as the Gobi Desert, Chile, Siberia, the Canadian Arctic, and the dry valleys of Antarctica—all of which are extremely remote and inhospitable, yet have provided McKay and his NASA colleagues with invaluable theoretical musings about the nature of life on the red planet.

“As you might imagine,” McKay continues, “the same question comes up time and time again during discussions about whether it’s necessary to send humans to Mars. I happen to think that it is necessary, since we won’t know exactly what we’re looking for until we get there. We’ll be depending a lot on the intuition of the scientists involved. They’ll know what’s important when they see it.”

McKay and Boston met as accelerated students at Florida Atlantic University in 1972 and have remained fast friends over the ensuing decades. Both have been fascinated with space since childhood. In 1976, when the Viking missions landed on Mars, that fascination crystallized into an infatuation with the fourth planet. “I remember the Viking landings as incredibly thrilling events,” Boston says.

“Viking told us very clearly that all the elements for life were present on Mars,” says McKay, “yet there was no life there—the lights were on, but nobody was home. That really piqued our interest. Was there life on Mars in the past? Could it harbor life in the future?”

McKay and Boston were both graduate students at the University of Colorado when Viking landed. In 1981, as a school project with a half-dozen other Mars fanatics, they put together an ambitious seminar christened the Case For Mars Conference, the primary goal of which was to advance an argument for the human exploration of the red planet.

“We were just a bunch of students,” Boston concedes. “We had no money, no influence. Basically, we started calling up famous Mars scientists and inviting them to our conference, and much to everyone’s surprise some big names like Conway Schneider and Ben Clark agreed to come.” It proved to be a roaring success, and was reconvened every three years thereafter. A passionate, if unofficial, network of Mars scientists and aerospace engineers coalesced around these parleys; dedicated to keeping Mars exploration on the national agenda, the group started calling itself the Mars Underground.

McKay is quick to point out that the Underground is not interested in putting a person on Mars merely as a grand adventure or symbolic act. They believe quite strongly that it should be more than an Apollo-like exercise in national chest thumping. “We don’t want to simply go to Mars, say ‘Hi!’ and come home,” he insists. “We hope to conduct serious research there: We want to know if there is life on Mars, or has been in the past. Imagine what it would mean to discover that we are not alone in the solar system? Such knowledge would have a profound impact on our understanding of life.”

***

Facing a head-high limestone boulder plastered top to bottom with gorilla shit, Penny Boston places four laboratory slides on a narrow ledge. On a subsequent trip she will collect them to see if any bacterial cultures have taken hold. Nearby, Chris McKay extracts a speck of fluffy, brownish-orange corrosion residue with a Swiss army knife and places it on a hand-held electronic pH meter. The gauge registers 1.3, prompting him to exclaim, “Wow, this stuff is acid. No wonder it’s eating up the limestone.”

Corrosion residue is typically found in relatively lofty reaches of the cave, where updrafts of warm air collide with cooler rock. According to the conventional wisdom, the stuff is the byproduct of an inorganic chemical reaction between humid, acidic air and the soluble limestone. The Mars Underground crew has an entirely different theory, believing corrosion residue to be biologic in origin. “It’s our hypothesis,” says Penny Boston, gesturing to the brown crud covering the rocks around her, “that all this nasty stuff has been produced by bugs—a population of microbes native to this cave.”

When Lechuguilla was first explored in the late 1980s, it was assumed to be virtually sterile. The rubble that had hidden the entrance from spelunkers had effectively sealed off the cave from the outside world. Even the water in the cave was free from surface contamination: analysis showed no trace of tritium, which meant that all the water now in Lechuguilla percolated into the cave before the widespread nuclear testing of the 1940s and ’50s.

As cavers ventured into the depths of the cavern, they came across the bones of a few stray bats, and even the fossilized remains of a prehistoric camelid (indicating that long ago Lechuguilla was accessible from the surface), but the deeper reaches of the labyrinth appeared to be completely devoid of life.

Then, in 1990, a government geologist named Kiym Cunningham made an astonishing discovery. While using a scanning electron microscope to study the mineral composition of a chunk of calcite collected in Lechuguilla, he noticed some peculiar, filamentous strands. “Kiym’s not a biologist,” says Boston, “but he’s a bright boy. He thought these things looked an awful lot like some kind of fossilized microbial life.”

Shortly thereafter, Larry Lemke’s wife was watching a National Geographic television special about Lechuguilla. “Larry,” she called to him in the next room, “you should come in here and take a look at this.” Lemke arrived in time to see a brief mention of Cunningham’s discovery. “A light bulb immediately went off in my head,” he says. There ensued a marathon cross-country conference call between Lemke, Cunningham, McKay, and Boston. The upshot was a NASA mission to search for microbial life in Lechuguilla.

Microbes may be tiny, but they have to eat—they need to draw on some source of external energy. “In most caves,” explains McKay, “the microbes you find eat organic stuff—bat guano, say, or other nutrients washed down from the outside. What’s so interesting about Lechuguilla is that it’s isolated from the surface, so there is no sunlight for photosynthesis, and no source of organic energy. Any bugs living down here have to be metabolizing inorganic minerals.”

The majority of the world’s limestone caves were dissolved from the surrounding bedrock by mild carbonic acid. Lechuguilla is fairly unique in that it was probably formed by sulfuric acid that percolated up from nearby petroleum fields as hydrogen sulfide gas. The copious deposits of gypsum in Lechuguilla were created as the sulfuric acid dissolved the limestone. Sulfur, also plentiful in the cave, was another byproduct.

McKay and his colleagues speculate that this abundance of sulfur serves as an important energy source for a complex microbial ecosystem, the most visible sign of which is the wealth of corrosion residue. The NASA folks believe that corrosion residue is organic waste produced by rock-eating autotrophic bacteria. It’s not gorilla poop that coats these cave walls, in other words—it’s germ poop.

McKay and Boston are now working to prove this hypothesis, building on earlier research by Kiym Cunningham and biologists Larry Mallory and Diana Northrup. On this and future trips into Lechuguilla, they will collect cultures of microbes from around the cave, identify them in the laboratory, and try to determine their relationship to each other. The results should ultimately help them with the question they’re really interested in: “Do bugs like this,” as Boston puts it, “also live on Mars?”

She and her colleagues admit that they are still a long way from proving that the corrosion residue in Lechuguilla is produced by microorganisms, let alone demonstrating that similar critters live on Mars. Back at the Deep Secrets basecamp, squatting in the dirt over a dinner of reconstituted chicken tetrazzini, Lemke emphasizes that “landing humans on Mars to prove that microbial life exists there, or once existed there, is a very daunting challenge. The round trip would take a minimum of 600-1,000 days. The technological obstacles would be huge.”

Even if the United States made putting a person on Mars a national priority and slated it for the fast track, says Boston, “it would take at least 15 years for it to happen. And right now Mars isn’t a priority. We’re doing this research on a shoestring.”

“I think we’re at a crisis point,” warns Lemke. “For a long time the space program was a way to conduct the Cold War by other means. We all got behind the Apollo program to beat the Russians to the moon, and as a cultural event it made the entire country feel great. Now that we’ve won the race, we have to ask ourselves if we’re willing as a nation to continue doing space exploration for its own merit. I think it’s an open question.”

Surprisingly, given their abiding passion for the red planet, Lemke, Boston, and McKay are in agreement that it would be a mistake to launch a crash program to put a person on Mars. “There are too many downsides,” explains Boston. “We don’t want to go through Apollo again—the goal becomes simply getting there. We want to achieve something more meaningful. We think it’s better to proceed slowly and thoughtfully.”

A go-slow approach is fine, says Lemke, “as long as you can sustain a public commitment to the program’s goals over the long haul. That’s very difficult in the face of so many competing demands for funding.”

“Which brings us back to what we’re doing in this cave,” McKay chimes in. “Part of our work is to establish a clear goal: to answer the question, ‘Why go to Mars?’ We think the best answer is, ‘To look for life.’ People can grasp that. It has universal appeal. By going to places like Lechuguilla—by digging up weird life forms and saying, ‘Look, folks, this is interesting stuff, and it could also be happening on another planet’—we hope to build enthusiasm for eventually going to Mars.”

Nobody in the Underground thinks getting to Mars will be easy (politically or otherwise), but they are patient people, and determined. The trip into Lechuguilla has only fanned the flames of their ardor. In 1998, NASA plans to land another unmanned spacecraft on Mars, and the folks in the Mars Underground are aggressively pushing to equip that lander with drilling equipment, enabling it to make at least a cursory probe of the planet’s subsurface environment.

Assessing the prospects for the Underground’s research efforts, Boston says, “All in all, we’re pretty ecstatic and fired up.” Nevertheless, she adds, “I think there’s a major object lesson in this for those of us planning the search for past or present life on Mars. It’s damn hard even here on earth, where we are awash in colleagues to consult and sophisticated laboratories to work in. We’ve only just begun to convince other scientists that there is indeed life in Lechuguilla. How then to convince them that there is life on Mars when we someday arrive there?”

Out in front of the NASA team, hours into the long ascent from the depths of Lechuguilla, I round a bend to encounter something so strange and unexpected that it takes me a few moments to recognize what it is: a cool breeze blowing across my grimy, sweat-drenched skin. A little farther and I arrive at something even stranger: a ray of sunlight, leaking exquisitely through a crooked fissure overhead. I am almost back to the surface.

The last remaining obstacle is an overhanging, 60-foot climb out of the guano-spackled entrance pit. Muscling up the final strand of free-hanging rope is exhausting, but I am so relieved to be escaping the underworld that I scarcely notice my labored breathing and cramping arms. Just before noon I pull over the lip and emerge into a brilliant New Mexico morning.

Sunlight washes over my chest and face. I inhale a greedy lungful of desert air, savoring the scent of juniper and sage. The colors that flood my light-starved retinas—the blue of the sky, a pale green drift of cactus, the creamy palette of the clouds—seem electric, surreal, almost overwhelming. An involuntary whoop of joy erupts from my throat. I feel as if I’ve just been released from prison.

Inebriated with a newfound appreciation of the ordinary, I imagine for a moment that I’m getting a taste of what an astronaut might experience on his or her return from a mission to Mars. Then I remember that I’ve been underground, away from the world, for a mere five days. My God, I wonder: If I’m this discombobulated by an absence of less than a week, what would it feel like to return to earth after a journey of two or three years?