Eavis, now the chairman of the British Caving Association, first came to China in 1982. The visit to its karst capital, Guilin, in the humid far southeast of the country, was a quick stop on his way back home from an expedition in Indonesia. The peak-ringed city and surrounding Guangxi region were then a different place: bicycles but few cars, peasants in blue work suits, escorts for foreigners. Eavis and his partner skipped much of the tourist program to meet with officials at the Institute of Karst Geology, the beginning of a relationship that would bring British and Chinese cavers into the limestone for the next three decades. The pace of discoveries would be nearly as rapid and stunning as China’s own transformation during those same years.

Eavis is in Guilin this time with a team of ten international cavers. When we arrive, we are met by a buzz of taxis and scooters in a city swollen to roughly a million people. The new China—freeways, luxury supermalls, peaks turned into quarries to feed the boom—is astonishing, especially to two team members. Richard Walters and Peter Smart were with Eavis in Guilin in 1985 and ’86, the first of two dozen expeditions that would make up the pioneering China Caves Project. Neither has been back until now.

China's Underground Kingdom Southern China holds the world's largest concentration of the eroded topography called karst. For more than 600 million years, this region was covered by a sea and accumulated miles-thick layers of sediments, including limestone. Uplift and erosion of the geological formation created today's massive caverns. MARTIN GAMACHE AND LAUREN JAMES, NGM STAFF. SOURCES: Andy Eavis, Tim Allen, Richard Walters, Peter Smart, Daniela Pani, Jane Allen, AND Mark Richardson, China Caves Project; Zhang Hai, INSTITUTE OF KARST GEOLOGY, GUILIN, CHINA

Walters, a telecom entrepreneur who has been known since childhood as Roo, will help operate the scanner along with Pani, who in past work discovered sunken WWII ships in the Mediterranean and helped support astronaut training in the caves of her native Sardinia. Smart, a noted karst scientist who retired from the University of Bristol in 2009, has a professorial beard and perpetually crooked glasses. He had to be persuaded to use a modern descender, a device to control his slide down a caving rope—his old one was just fine, thank you. But he is excited by the innovation that is laser scanning, also known as lidar, because otherwise “caving is through a glass darkly.” For all of China’s change, we find that as we continue west from Guilin toward the biggest cave chambers, Smart’s expert description of the landscape still fits. “From above,” he says, “it looks like an egg box.”

Cedar Wright, Matt Segal, and Harrington ascend a limestone spire in Enshi canyon. The Stone Forest is a labyrinthine world of eroded and dissolved limestone near the southern Chinese city of Kunming. Early visitors gave the formations names like “rhinoceros admiring the moon” and “stone singing praises of plums.” Grabbing the rock high above a bend in the Getu He river, Matt Segal scales the Great Arch of Chuanzhang Cave. Tourist boats often ply the river below, providing a less challenging view of Ziyun Getu He Chuandong National Park. Wright balances atop a spire in the Stone Forest. Deposited in a shallow sea 270 million years ago, the limestone here was eroded by heavy tropical rains.

By area, Hong Meigui Chamber, the first we plan to scan, is thought to be roughly the size of nine football fields. It is number eight on a 2012 list that cavers have been passing around of the world’s largest known cave rooms, behind contenders in Malaysia, Spain, Oman, Belize, and elsewhere in China—but where it ranks by volume is a question we hope to start answering in 3-D. Our base for this early part of the expedition is not a subterranean camp but an aging, industrial-size hotel in the town of Leye, which had 5,000 people when first visited by the China Caves Project. Now it has many times that, and more than 160,000 tourists a year come to see the nearby Dashiwei Tiankeng, a 2,000-foot-wide, 2,000-foot-deep sinkhole first known to Karst Institute scientists in 1998 and explored by the China Caves Project two years later. A photo of Eavis is displayed in the local museum.

To reach Hong Meigui each day, we drive to a parking lot not far from town, where we change into coveralls and don harnesses and helmets and headlamps, then walk for a minute or two to an unremarkable-looking opening in the forested mountainside. Past a concrete cistern farmers use to collect water dripping from an overhanging roof, the cave swiftly becomes colder and steeper and darker. Soon enough, we are in another world.

Two short rappels—maybe 15 feet and 50 feet—have been rigged by the group’s two ablest cavers, Tim Allen and Mark Richardson. Otherwise the approach is on foot. For much of an hour on my first descent, I follow Tim’s wife, Jane Allen, another expert caver, down a staircase of pools shimmering in our headlamps and into a tubelike passage where the surface looks like—and sometimes surely is—a river of mud.

The sensation when we enter Hong Meigui Chamber itself is both dizzying and familiar. I can see that it is big simply because I can’t see much at all; no longer does my light bounce off a ceiling or walls. Particles float in the air, for not even the wind can reach here. A boulder the size of a dump truck has fallen to the floor from someplace dramatically high above, its crater ringed by a shock wave of mud; the team names it “the meteorite.” Somewhere far on the other side of the room—exactly how far is hard to gauge—the beam of someone’s lamp bobs along. Only when I begin scrambling up a rubble slope does the experience seem familiar. The slope is so big, the progress so gradual, the terrain so rough, it feels like mountain climbing on a starless night.

Coming into view Three-dimensional scanning reveals the incredible scope of the Miao Room and the world hiding inside.

Given the irregular shapes of caves, it can be hard to decide where each room ends, where to draw the boundary lines. What constitutes a cave chamber, and what a mere passage? This semantic question will be constantly argued over by expedition members, for one of the eventual goals of 3-D scanning—a ranking of the world’s largest chambers by volume—is impossible if cave explorers can’t agree on a definition.

The largest known is Sarawak Chamber in Malaysia, which Eavis and two others discovered in 1980 and helped scan in 2011. Its estimated volume is 338 million cubic feet—more than three times the size of the new Dallas Cowboys stadium in Texas. My layman’s answer to the definition question is that we never argue over whether something is a full-blown hall or a hallway; you know it when you see it. In the case of caves, you can’t entirely see it. But you know it: There is the unmistakable impression of empty space. None of the cavers find this answer satisfying in the least.

When I catch up with the scanning team, they are in the cracked mud near the meteorite, not far from the edge of the lake and the sheer face of a limestone wall that leads to the hidden roof of the cave. This is one of 17 scanning stations in Hong Meigui—so many because a laser scanner can see no better around a corner or a boulder than a human can. The scanner emits laser pulses and measures how long it takes for them to be reflected back. Distances are easily determined based on the speed of light. Our model is a Riegl VZ-400, used in architecture, engineering, and mining and now for the first time in caving. It’s a metal cylinder about the size of a human head and weighs 21 pounds, not including its two nine-pound batteries or the tripod or laptop and cables. When running, it sits at about eye level, spinning 360 degrees and taking up to 122,000 measurements per second of everything within a maximum 2,000-foot radius.

Caver's Paradise Some of the largest caves in the world are found in the thick limestone rock underlying Guangxi and Guizhou Province. At one end of the Gebihe cave system is the Miao Room, part of a large underground stream course, which ranks as Earth’s second largest known cave chamber by area, after Sarawak Chamber in Malaysia. JUAN VELASCO, NGM STAFF. SOURCES: Andy Eavis, Tim Allen, Richard Walters, Peter Smart, Daniela Pani, Jane Allen, AND Mark Richardson, China Caves Project; Zhang Hai, INSTITUTE OF KARST GEOLOGY, GUILIN, CHINA

To set up the scanning station, Walters uses a pocket level to ensure that the tripod is true, orients the scanner with a compass, then pulls a new 17-inch laptop out of a waterproof case and hands it to Pani, who sits in the muck with the computer on her lap. Eavis stands nearby, shuffling bags out of view and generally trying to hasten the process—the faster this cave is scanned, the more chambers we can visit—to the frequent annoyance of the meticulous Pani. They attach a blue-green Ethernet cable to the laptop and push a button on the laser scanner, and suddenly it comes alive, its head silently revolving as the team seems to hold its collective breath.

Three minutes later the results appear on Pani’s laptop. The rendering is in black and white and low resolution. But it is stunning. There, as we crouch in the dark in the mud, staring at the bright screen, Pani flies us through the virtual cave—and I can finally see where I am. It’s an out-of-body experience.

As the expedition moves on to two other great chambers, Miao and Titan, we are reminded that Hong Meigui is an oddity in China for reasons beyond its scale. First explored in 2001 by foreign cavers, the chamber did not have a single human footprint until they arrived—perhaps because the two cliffs at its entrance discouraged locals. Many caves in southern China have a human history that dates back to at least the Qin and Han dynasties, two millennia ago. Underground studies during that time were in the pursuit of chi, or life energy, which the karst regions were believed to possess in great abundance. Stalagmites and rimstone pools also provided ingredients for early aphrodisiacs and medicines; cave chambers became places for prayer. Even today farmers use cave entrances to store and dry grain.

A waterfall hundreds of feet underground in Hong Meigui—one of the largest cave rooms in the world—signals recent rainfall on the surface above. Over centuries, the gradual drip of water has helped form the caverns’ stalagmites, like that on the left. Members of a British-led expedition pause at a subterranean lake on the way to Titan Chamber in southern Guizhou Province, where it rains more than 50 inches a year. The lake appears and disappears as the rains come and go. Uphill from the giant Miao Room, 21 families from the Miao minority live under the roof of a cave. They first came, elders say, because of the reliable spring. The cave now contains a basketball court and, until recently, had a school. Crown Cave on the Li River, first surveyed by a British-Chinese expedition in 1985, has been transformed into one of the Guilin area’s top tourist attractions. Inside are trinket sellers, underground train rides, and wishing wells that attract thousands of visitors at a time. Yangshuo was a sleepy village two decades ago, when foreign climbers and cavers began frequenting nearby formations. Today domestic tourists vastly outnumber foreign visitors.

On our way to Hong Meigui, we had stopped in Fengshan, eight hours west of Guilin and part of the new, 360-square-mile Leye-Fengshan Geopark. Here was a large municipal cave, Chuanlongyan, that enclosed a two-lane road, an open-air museum, and a public amphitheater. When I strolled down the road one afternoon, a young couple had stopped their motorcycle in a far corner of the museum parking lot and were kissing in the darkness. “This is the best use for a cave,” admitted the geopark’s resident foreign expert, French caver Jean Bottazzi.

Bottazzi, Eavis, and Smart showed rough scans of local caves to a regional official in Fengshan. He immediately wondered if they could tell which sections of a cave were unstable. Eavis, successful at caving in good part because he’s successful at navigating officialdom, picked up the utilitarian thread. “Yes, of course,” he said. Smart added, “You could rope off any dangerous areas, so tourists keep to a safe path.” The karst region’s tourism boom—fueled by China’s growing middle class and a nostalgia for iconic landscapes—is on everyone’s mind.

In Fengshan we also saw families in orange life jackets pushed by boatmen up an aquamarine river, hooting as they floated by stalactites in low-hanging caverns. Ten hours to the north, Ziyun Getu He Chuandong National Park is already attracting rock climbers. When we arrive from Leye and Hong Meigui, workers are drilling a tourist footpath into the tall walls of Yanzi Cave, named for the swallows that nest in those same walls. It leads to a new elevator. In Getu we scan what is considered to be the planet’s second largest cave chamber by area, the Miao Room, as big as 22 football fields.

Segal and Wright descend from the Great Arch in Ziyun Getu He Chuandong National Park at sunset. A new glass elevator delivers visitors to the foot of the arch. “China just has a different approach to things,” Wright said. A boatman guides tourists down the Poxin River as it emerges from underground. The river is part of an extensive geopark that also includes Dashiwei Tiankeng, a 2,000-foot-deep collapsed cave. What is the popular Li River worth to everyday Chinese citizens? At least $3.20: Its scenery graces the back of the 20-yuan note. Sightseers on large tour boats or small bamboo rafts take daylong tours from near Guilin downstream to the tourism boomtown of Yangshuo.

One day I walk with Eavis and Pani to see an entire village housed in a 600-foot-wide cave nearby. Twenty-one families inhabit roofless bamboo homes, and they have a basketball court, a shuttered primary school—and a small but growing stream of visitors. Enough tourists every week, we are told, that officials now pay the cave dwellers to stay put rather than move to modern homes outside it.

Before the drive south from Getu to our final scanning objective, Titan Chamber, American expedition member Michael Warner tries to make sense of what it is we are doing here. Every chamber we visit has been visited before, he notes, if not by cavers then by farmers—so this is not discovery. “Exploration is just documenting something for the first time,” Warner decides. “And laser scanning is the best way yet devised to document a cave.”

If there is a perfect cave for the fledgling art of subterranean laser scanning, Titan is it. At the center of its massive chamber, slopes covered with rubble and pockmarked with pools creep relentlessly up to twin, 50-foot stalagmites that sit on the very peak of an underground mountain. Place the scanner atop the big one on the right, and you can take in almost all of Titan—about 13 acres, an area slightly larger than Hong Meigui—in a single 360-degree sweep. Past the high point there are more stalagmites, a formation that looks uncannily like the head of a crocodile, teeth included, and an underground lake that dries into a bed of cracked mud while we’re there.

The Karsts of China Climber Cedar Wright shares his passion for exploring China's ancient rock formations.

When all return to the surface, dirt-streaked and weary, a provincial official is waiting. “Is it the biggest chamber in the world?” he asks. A “yes” would change everything for the local economy. But it is not the biggest in the world. Perhaps it is in the top ten. The list by volume is still being made. The official is disappointed. “But it is one of the most beautiful cave chambers I have ever seen,” says Eavis.

We think the expedition is done, but Eavis has a surprise for us the day before we fly home: a cruise through the karst down the Li River, Guilin’s top tourist attraction, with a stopover at a cave his team was the first to survey in 1985. He did the cruise in 1982, back when there were a few dozen riverboats. Now there can be a couple hundred a day, each of them carrying a hundred tourists, and thousands of people flood Crown Cave.

The Li River is still beautiful, but Crown Cave, after Titan, is jarring. We are herded into the entrance in groups of 20, each following a guide with a microphone and cheap portable speaker who yells to make her voice heard over the cacophony of other guides. Inside, the stalagmites and pools are lit with gaudy green, red, and purple lights. There are paths and handrails and, in some of the chambers, trinket stands. Partway through the cave is a glass elevator. Our guide hurries us to get in line for the underground train, which will take us to the line for the underground boat trip, which will take us past the underground roller coaster and across the bridges over the underground river.

Eavis hangs back, snapping photos of everything. He was once alone in Crown Cave, mapping it, an explorer discovering hidden passages. Now this. We start jogging up the steps of the path to catch up with our group. “Is this disorienting?” I ask him. “Nah,” he says, and he keeps jogging. The tourists are now pulling out cameras of their own, documenting every last corner of Crown Cave that’s visible in the artificial light—an exploration of a sort. To Eavis it’s the most natural thing in the world.

McKenzie Funk’s new book is Windfall: The Booming Business of Global Warming. Carsten Peter’s photos of Vietnam’s caves appeared in our January 2011 issue.

Society Grant: The British-led cave expedition and the rock climbing expedition by Cedar Wright, Emily Harrington, and Matt Segal were both funded in part by your National Geographic Society membership.