This story appears in the March 2017 issue of National Geographic magazine.

‘Don’t worry, you can’t get lost down here.’ Larisa Pozdnyakova’s words, in her thick Russian accent, float to me from within the cave’s seemingly endless black void. Apparently, she can read my mind: All I can think about is not getting lost a mile inside the Earth. For the past several hours I’ve struggled to keep up as she leads me deeper into a frozen underworld known as Dark Star.

Larisa, a 30-something veteran caver from the Ural Mountains, moves with fluid, snakelike ease along our twisting route, while I grunt and heave my way after her like the clumsy rookie that I am. The cold blackness swallows the light of our headlamps just a few feet from our heads, forcing us to move like moles, scuttling, slithering, feeling our way along hundreds of feet of stiff, mud-caked ropes that help guide us through myriad passages known in caving argot as “squeezes,” “meanders,” and “shafts.”

These passages have already been mapped, but as we crawl up and down, side to side, I feel disoriented by the nightmarish spiral of icy mud and wet gravel. For a climber and mountaineer like me, this is an altogether different kind of navigation. I’m accustomed to moving across dangerous terrain, but down here printed maps are often useless, GPS doesn’t work, and there are no celestial guides to offer reassurance. And despite what Larisa tells me, I know I could never find my way out of this soul-sucking labyrinth on my own.

When I finally catch up, she has stopped at a ledge overlooking what our headlamps reveal to be a body of water—one of Dark Star’s many subterranean lakes. She grabs a lanyard attached to her harness and clips it on a gritty rope attached to a bolt hammered into the rock above us. The rope leads out over the lake and disappears into the black. The setup acts as a sort of zip line to ferry cavers across the frigid lake, too cold to swim in without a wet suit. She gives me a perky smile and steps off the ledge. Her blond ponytail whips wildly in the beam of my headlamp before she vanishes into the darkness, leaving me alone with my fears.

View Images Outside the cave, it’s a blistering 100°F. Inside, temperatures range from 30°F to 37°F—a small variance with a big impact on the scenery: As team members descend deeper, blue ice gives way to barren rock.

I'm in this predicament because I signed on with a 31-member expedition—composed mostly of non-English-speaking Russians—to explore this monstrous limestone cave system inside a mountain in a remote corner of Uzbekistan. The Russians spotted an entrance to the cave in 1984, but British cavers were the first to reach it and began exploring the system in 1990; they named it after a satirical American sci-fi movie from the 1970s. In the decades since, Dark Star, along with neighboring Festivalnaya (the two systems may someday be found to be connected), has drawn hard-core cavers from around the world.

The allure of this huge system is similar to that which big mountains hold for climbers—with one difference: We know that Mount Everest is Earth’s highest peak, but the potential for conquering new and enormous subterranean voids is almost limitless. More is known about the terrain of Mars than about what lies hidden beneath the Earth’s surface. Krubera Cave in the republic of Georgia is currently the deepest known cave, at 7,208 feet. But Dark Star, with so many areas still to survey, is a prime candidate to take over the title.

To date, eight expeditions have identified nearly 11 miles of Dark Star’s passageways, the deepest lying about 3,000 feet below the surface. But the system hasn’t been fully mapped, partly because of its remote location in a politically unstable region and partly because its vastness requires advanced technical abilities and a lot of equipment. Many expeditions have simply run out of rope. I can immediately see why. Just a thousand feet from our entry point, Larisa and I had already negotiated nearly a dozen roped sections.

She and I had been paired at base camp: her assignment, to guide the “Amerikanski” (I’m sure I heard them calling me that) to Gothic camp, more than a mile inside the mountain. I would spend two nights recording the team’s progress in mapping new parts of the cave and collecting scientific data.

View Images Ice crystals populate Full Moon Hall. The chamber, 820 feet long, is the largest yet discovered in Dark Star. The entire cave system is a geological time capsule: Mineral deposits reveal millennia of climate history.

Never mind the perilous trip following Larisa to Gothic camp—the aboveground journey to our base camp at the foot of the mountain was no walk in the park either. To meet up with the expedition team—an ensemble of world-class cavers and scientists ages 22 to 54 that, in addition to the Russians, included Italians, Israelis, and one German—I traveled to Tashkent, the capital of Uzbekistan. From there we traveled together a little over a hundred miles by bus, with hundreds of pounds of food and gear for three weeks in the field, across the arid plains. We took a popular tourist route that follows the ancient Silk Road to Samarqand. Then we turned off the beaten path, heading south toward the Afghan border to Boysun, where we loaded everything into a six-wheeled Soviet-era troop transport.

As we lumbered into the Boysuntov (also known as Baysun-Tau) Range, the mountains gradually rose to 12,000 feet and then dropped off in a jagged line of spectacular cliffs. In the deep valleys between we could see a hodgepodge of small villages where Tajiks and Uzbeks have lived for centuries, herding goats and harvesting watermelons, plums, apples, and walnuts and fetching water from springs fed by the underground rivers that perforate these mountains.

It was some 30 years ago that Igor Lavrov, the heavily bearded, bespectacled geologist who now sat across from me in the back of the truck, discovered the towering limestone cliff called Xo‘ja Gurgur Ota that he and his fellow cavers are still exploring all these years later. This wall, 1,200 feet high and 22 miles long, was formed when tectonic forces thrust ancient beds of limestone into vertical walls of rock. Igor was a 24-year-old junior member of the Sverdlovsk Speleological Club, which had learned about the Boysuntov by studying old Soviet geologic maps. One day, following a tip from an itinerant shepherd, he and a companion, Sergei Matrenin, met the schoolmaster of a small village named Qayroq. The man had spent years exploring nearby grottoes with homemade torches. “Where can I find these caves?” asked Igor. “There,” said the schoolmaster, pointing to the monolithic limestone wall at the head of the valley. From the bottom of the rock face, the two cavers first spotted the mysterious hole halfway up the cliff that was to be our entrance to Dark Star.

View Images A team member rappels down the face of a limestone cliff to investigate one of two large portals that could reveal a new entrance to Dark Star. Hopes were dashed; both holes led to solid ice.

Once the route became too steep for the truck, we hiked for two days with 15 donkeys to haul our supplies up to the base camp, perched on sloping terraces at the foot of the limestone escarpment. All of Dark Star’s seven known entrances are found on this face and can only be reached via technical climbing or rappelling.

It took us several days of rigging ropes to access the cave and haul up gear. But finally I hoisted myself up a 450-foot rope to the cave’s main entrance (dubbed Izhevskaya, or R21). I began to see why cavers think of Dark Star as a living, breathing entity. Down at base camp, the temperature hovered around 100°F, but up here I was shocked to find myself bracing against a freezing wind blasting out of Dark Star’s mouth.

No one fully understands the cave’s ventilation system, but this particular entrance “exhales” when the barometric pressure outside is high and “inhales” when the pressure is low. If Dark Star was exhaling here, it must have been sucking in air somewhere else. But where? As I scurried down a frost-covered slope into the cave, I couldn’t shake the distinct feeling that I was stepping into the maw of a prehistoric beast.

CAVE CRAWL On hands and knees, amid pools of water, Mark Synnott inches his way through a narrow tunnel in the cave system. Watch him navigate the shaft.

Just inside the entrance, Tonya Votintseva, a Russian molecular biologist, stopped to attach a small white disk to the wall. Her official assignment is to map any newly discovered areas of the cave, but she admits she is more interested in science than exploration. This data logger is one of several she will install throughout the cave to record the temperature, humidity, carbon dioxide level, and barometric pressure for the next two years. Then they’ll be collected and taken to a lab for analysis.

A lot of science can be gathered underground, much of it contained in speleothems—mineral deposits called stalagmites and stalactites that rise from cave floors and descend from the ceilings. In the same way that scientists use ice core samples taken from glaciers, they can gather data from speleothems. By analyzing the chemical components delivered to these formations by drip water over millennia, they can get clues to Earth’s climate at various points in time.

Each year the team collects samples from different parts of the cave system to gain insight into not only the climate history of Central Asia but also the cave’s ventilation system and architecture—knowledge that helps future cavers determine where to find promising new passages to explore. Following Tonya, I duck under an archway of translucent blue ice and enter a massive chamber some 820 feet long and a hundred feet tall—the Full Moon Hall. Turning my headlamp to its full brightness, I pan across the room. The walls are covered with delicate feathers of hoarfrost that blink in the light like millions of tiny mirrors, illuminating the hall like galaxies of stars in a crystal clear night sky.

View Images Zhenya Tsurikhin climbs a free-hanging rope. Dark Star’s passages are deep underground, yet many are 10,000 feet above sea level. The thin air at that altitude ups the physical challenge of negotiating the cave.

Two days later I am at the edge of a lake with Larisa, who’s out of sight, waiting for me on the other side. At least I hope she is. Since I joined the team, the Russians have seemed intent on reminding me of my rookie status, telling campfire stories of cavers who met with tragic ends, including a young explorer who made a wrong turn and got lost in a cave in Britain. “One year later they found his body,” one of them tells me. They’ve also been poking me with random challenges that seem designed to determine whether the Amerikanski can hang with them—seeing how heavy a load I can carry, how good my rope skills are, how much I’ll let them screw with me.

There’s only one thing to do. I clip my harness to the rope and slide to the other side of the lake, touching down on a ledge that leads into a small domed chamber roughly the size of a large igloo.

Larisa is not there. The current challenge seems to be to see whether I can find my way alone. So far, I’ve met their tests with competence and a good-natured laugh. But I’m not laughing now. A quick pan with my headlamp reveals two passages that spoke off from the chamber. I strain to hear any noise that might reveal which one Larisa has disappeared into, but all I can hear is the sound of water dripping from the ceiling into the lake.

How Deep Is Dark Star? Uzbekistan’s remote Boysuntov Range is hard to access, and the region is politically unstable. Still, the mysteries of Dark Star and nearby Festival- naya—two of the world’s deepest high-altitude cave systems—are an enduring lure for explorers. Europe Asia Uzbekistan The Boysuntov Range Like the Himalaya, the Gissar Range and its cave-rich spur, the Boysuntov (illustrated below), were forced upward by the collision of Indian and Eurasian plates. Kazakhstan Tashkent Kyrg. Samarqand Turkm. Cave Location Taj. China Boysun AfGhanISTAN Iran Pak. India A Rugged Approach Just reaching Dark Star’s seven known entrances (five shown here) is tough: They can’t be attained without rock-climbing skills and equipment. No End In Sight Dark Star’s deepest known point is about 3,000 feet below the surface, where limestone meets the underlying insoluble rock. But unexplored branches could plunge much deeper. Peak of the Three Brothers 12,306 ft 3,751 m Plateau camp Cave entrances Orenburg Metro Dark Star Red Dwarf Ice Maiden Orenburgskiy * Frozen Beck Red Wine Izhevskaya ** Full Moon Hall *Highest point of cave Gubahinskiy **Expedition entrance Broadway Passage discovered during the expedition Gothic camp White Bear series Red Lake series Waterfall series Velvet Gallery Festivalnaya 1.1 mi, 1.8 km Bottom Lowest explored point of the cave: 8,698 ft, 2,651 m Scale and direction Vary in this perspective. How Deep Is Dark Star? Kazakhstan Uzbekistan’s remote Boysuntov Range is hard to access, and the region is politically unstable. Still, the mysteries of Dark Star and nearby Festivalnaya—two of the world’s deepest high-altitude cave systems—are an enduring lure for explorers. Tashkent Kyrg. Samarqand Turkm. China Cave Location Taj. Boysun A Rugged Approach AfGhanISTAN Just reaching Dark Star’s seven known entrances (five shown here) is tough: They can’t be attained without rock-climbing skills and equipment. Iran Pak. India Europe Asia The Boysuntov Range Uzbekistan Peak of the Three Brothers 12,306 ft 3,751 m Like the Himalaya, the Gissar Range and its cave-rich spur, the Boysuntov (illustrated below), were forced upward by the collision of Indian and Eurasian plates. Dark Star Plateau camp Red Dwarf Orenburgskiy (highest point of cave: 11,778 ft, 3,590 m) No End In Sight Orenburg Metro Dark Star’s deepest known point is about 3,000 feet below the surface, where limestone meets the underlying insoluble rock. But unexplored branches could plunge much deeper. Red Wine Po-Sokolovski Ice Maiden Frozen Beck Izhevskaya (Expedition entrance) Full Moon Hall Gubahinskiy Festivalnaya Passage discovered during the expedition This system’s 10 miles of chambers and passages descend 2,051 feet—possibly more if a link to Dark Star is ever found. Broadway Cave entrance Katta Tash Gothic camp 12,218 ft 3,724 m White Bear series Red Lake series Hatched area shows the approximate location of Festivalnaya; 3-D survey data is incomplete. Waterfall series Velvet Gallery Connecting the caves At their closest points, the Dark Star and Festivalnaya systems are separated by just 1.1 miles. Bottom Lowest explored point of the cave: 8,698 ft, 2,651 m Scale and direction Vary in this perspective. Charles Preppernau, NGM Staff; Sources: Antonina (Tonya) Votintseva and Zhenya Tsurikhin, Ekaterinburg Speleological Club; Paz Vale, Eldon Pothole Club. SATELLITE IMAGE: DIGITALGLOBE

As I contemplate my options, I turn off my light to conserve the batteries. The blackness that envelops me is absolute. Photons of light travel billions of miles through the universe in straight, unobstructed lines, but they cannot bend. The twisting path that leads deep into the mountain restricts the only light that will ever shine on these walls to the beams of headlamps. I think about how the lost British caver must have felt as his lamp died, lying alone in what would become his tomb.

“LARISA!” I yell, but the sound just bounces off the walls in the tiny chamber. It suddenly becomes clear: Her “don’t worry, you can’t get lost” thing is some kind of insiders’ joke, because actually you can, quite easily.

The first passageway I follow turns quickly, mercifully, into a dead end. The second one leads me to a ledge of glossy flowstone formed by thin sheets of minerals deposited by a consistent flow of water. Larisa is sitting on it.

GETTING THERE Experience the cavers’ grueling journey to Dark Star—by truck, by donkey, on foot, with rope—as they sing a Russian caving song around the campfire.

On we go to a T-shaped intersection where two brightly colored tents, glowing with light, sit atop a pile of jumbled boulders: Gothic camp. A headlamp beam bobs in our direction and the voice of Zhenya Tsurikhin booms: “Welcome to Gothic Chamber.” Zhenya is the group’s elder statesman, on his 10th caving expedition to the Boysuntov. He breeds fish for a Russian state institute, but Dark Star is his true life’s work, and no one understands the cave’s complex networks better than he does. “He knows where new passages will lead before they are explored,” one of the younger Russians tells me.

Zhenya gestures toward one of the tents. Steam pours from its opening, and I can hear a stove purring inside. I slip out of my coveralls and follow him into the tent, where a few team members are huddled around a map of Dark Star. Passages discovered during each expedition are rendered in different colors, and the map looks like a multicolored schematic of the human circulatory system. Tracing a sinuous green line with a muddy finger, Zhenya taps a spot and begins speaking rapidly in Russian. He’s pointing to where the previous expedition ran into an impasse at a 120-foot waterfall. It has yet to be climbed.

I spend my first night deep in the bowels of the Earth, jammed into a tent with two other team members. Down here, day and night are irrelevant, and the team comes and goes, sleeps and eats, on a schedule unhindered by the position of the sun. I awake to the loud arrival of three Israeli cavers who have spent four days worming through a rubble-filled crack at the bottom of the cave. One of them is Boaz Langford, a young geologist who tells me he thinks they’ve reached the nonporous rock underlying the limestone. “We need to find a new direction,” he says. “We are going to explore the Red Lakes. You should come with us.”

View Images To camp inside the cave, the team pitches two tents to accommodate up to 12 people. At this point, more than a mile inside the mountain, the temperature of the cave is no higher than 37°F, so to keep warm, the team cooks food and boils water for hot drinks inside the tents. They also zip sleeping bags together to conserve body heat while resting.

Instead of waiting for me to suit up, he rattles off some quick instructions and is gone. Half an hour later, I am alone in the dark again, facing another fork in the road. There are two ropes: One drops straight down through a slot in the floor; the other angles upward and traverses an abyss—a deep pit or possibly a lake, I can’t tell—and disappears into a hole 20 feet above me. I opt for the slot in the floor and descend between overhanging walls of rippled orange flowstone to find another intersection of three passages with no indication of which way the Israelis might have gone.

I pick the least worst option: a tube about the size of an air duct filled with four inches of water. I shove my backpack in and nudge it forward with my head. I hold my torso out of the water by perching on my forearms and toes, inching forward in a gut-crushing plank position. The ceiling lowers until I’m forced to slither on my belly. Suddenly the tube turns almost straight down. It’s so tight that just flexing my muscles keeps me from diving down the shaft.

As the blood rushes to my head, another caving horror story comes to mind. A young American medical student was exploring a virgin passage in Utah’s Nutty Putty Cave in 2009 when it suddenly took a downward turn. He dropped in headfirst, assuming it would eventually open up. Instead it got tighter, and he ended up trapped upside down. Rescuers found him and were even able to get food and water to him as they worked. They almost got him out, but their equipment failed. They weren’t able to extract his dead body, so the passage was filled with concrete.

I am more fortunate, and when the tube spits me out into a water-filled corridor, I hear the sound of cave suits scraping against rock. I’ve found the Israelis. And they have found another small hole that drops even farther into the unknown depths of Dark Star. They are arguing over who gets to go in first. “It’s mine,” one says in Hebrew as he shoves his friends aside and dives into the hole.

View Images Perched on slippery rock, author Mark Synnott knows that falling into icy water isn’t an option. Here, wet clothes won’t dry. Hypothermia, a broken ankle, or getting lost are just a few of the risks that loom in Dark Star.

View Images Team members settle down in base camp at the foot of the Boysuntov cliff. Stars leave trails in the sky as the Earth turns; the photographer created this effect by leaving the camera shutter open for 40 minutes.

As time runs out on the expedition, most of the hoped-for new passages have proved to be dead ends. The team has exited the cave and is preparing for the long journey back to Tashkent, but Zhenya, along with an ambitious young Russian named Aleksey Seregin, insists on making one more push to climb the big waterfall and find a new passage.

When they finally return to the base camp, where we’re still waiting for them three days later, they are coated with grime and brimming with the news that they climbed the waterfall and after hours of shimmying in tight meander, it pinched down to a slot, barely nine inches wide. Aleksey tried to enter the fissure, but his head simply wouldn’t fit. Refusing to give up, Zhenya tried, jamming his head into the crack, his temples scraping against the icy rock. Tilting his shoulders and sucking in his belly, he wormed up a twisting chimney. After 30 minutes of contorting himself to move inch after painstaking inch, he finally popped through the crack into a passageway as big as a Moscow subway tunnel and reverberating with the roar of a fast-flowing river.

Was this the passage that he’d been seeking for more than 20 years? The one that will finally reveal Dark Star to be the Everest of caves? He desperately wanted to keep going, to see where it would lead. But alas, the expedition’s time had run out.

As the men relate their story, the jolt of the new discovery pulses through the team, and it becomes clear, even to the Amerikanski, that this is exactly how great caving expeditions should end: with the discovery of a mysterious passage snaking into the unknown—and the promise of a new adventure waiting deep inside the Earth.