Limits of the Known

by David Roberts

  All through the untamed jungles of the Yucatán, occasional interruptions of the monotony burst to the surface in pools of crystalline water. They are called cenotes, poorly translated as “sinkholes.” For the Post-Classic Maya, they were not only vital sources of water, but portals to Xibalba. At Chichén Itzá the tour guides take you to the famous cenote, where, they blandly inform you, the ancients sacrificed virgins to the gods. A cenote can range in size from a tiny lake down to less than a puddle. Because of the rain forest, thousands of cenotes have yet to be discovered by humans—or, at least, by humans in our modern age.

  Yet the cenotes are not isolated from one another. Underground, underwater, the limestone opens into passages and cavities that spread in all directions. The whole of the Yucatán, in fact, amounts to a huge sponge cake of cave systems, virtually all of them filled with water. Some of the systems even connect with the sea. They remain, in the aggregate, one of the least well-explored places on earth.

  The only way to discover these hidden sanctums is to cave dive. And cave diving, a “sport” still in its relative infancy, happens to be one of the most dangerous forms of exploration yet concocted. Cave divers must wed the complex crafts of scuba diving and caving. During the formative years of the endeavor, practitioners learned about the insidious perils of their exploit by trial and error, and an inordinate number of them died as a result of their mistakes. Even today cave diving remains extremely dangerous, and the death toll embraces not only novices but some of its most skilled devotees.

  In an underwater cave, divers cannot depend on simply “tagging” their progress with tape or other markers. Instead they trail guidelines—nylon cords regularly tied off to natural features as they swim through tunnels and holes and traverse massive rooms. In theory, the diver need only follow his guideline back to the entrance pit. As they lay the line, divers try to avoid snagging it on what are known as line traps. If the cord becomes trapped under a ledge, and a diver cannot flip or jerk it loose, he faces the terrifying option of abandoning the guideline as he tries to find its continuation on the other side of the ledge under which it’s jammed. Another hazard is the notorious silt-out. Not only the floors but the walls and ceilings of underwater caverns are often coated with a fine powdery sediment. The slightest disturbance can stir all that dormant silt into a local hurricane that takes many hours to settle out. A diver trapped in a silt-out is at least as blind as a mountaineer or pilot ensconced in a whiteout—or as blind as a dry caver who loses all his sources of light. It is virtually impossible to retrace one’s journey underground without a guideline, by feel alone.

  Add to these nightmare scenarios all the usual hazards of scuba diving—equipment failure, improper decompression on an ascent from deep water, the narcotic trance induced by “rapture of the deep”—and you can see why cave diving stacks up today as a pursuit as perilous as any form of terrestrial exploration. In 1994, inside a deep cenote in Mexico called Zacatón, forty-five-year-old Sheck Exley, widely regarded as the savviest and most experienced cave diver in the world (with twenty-nine years of service spanning more than four thousand dives), perished when he failed to resurface from a push intended to reach a near-record depth of 1,000 feet. The cause of Exley’s demise remains a mystery more than twenty years later.

  No region on earth promises a more complex network of underwater caves than the karst of the Yucatán. The first divers to explore the cenotes soon discovered that few of the crystalline pools were self-contained: instead, they branched in all directions, connecting to other cenotes and to mazes of watery passages with no outlet to the surface. The more they probed, the more these pioneers recognized how limitless the subterranean voyages they could perform might be. During the last twenty years, a small gang of adventurers has focused on exploring the cenotes of Yucatán. None of them is more passionate or articulate than Sam Meacham.

  Fifty years old, Meacham has been cave diving for more than a quarter century. His interest in scuba diving led him to move from the U.S. to the Mexican state of Quintana Roo in 1994. This in turn led to his fascination—nearly an obsession—with Yucatán‘s cenotes and underground rivers. A pet peeve of this seasoned explorer is his quarrel with the general perception that cave divers are, in his words, “a bunch of cowboy adrenaline junkies out there rolling the dice, leaving it to chance, cheating death on every dive.” Yes, he acknowledges, cave diving can be dangerous, but “the majority of fatalities . . . involve people who lack common sense, training, and the proper equipment. In the end, they made a horrible decision that ended their lives and, in many cases, the lives of others.”

  The hazards of cave diving, Meacham insists, can be controlled by proper training and judgment—not to mention an intangible skill he calls “situational awareness.” Among the iron-clad rules he now follows is never to dive solo. At the same time, Meacham practices a “small team approach.” As he explains, “The fewer cooks in the kitchen, the better. Suppose we have to hike three-quarters of a mile from the vehicle to the cenote. Each diver requires about 220 pounds of equipment. More people equals more gear equals more of a logistical headache. We may have a team of four or five people in the field that day, but only two of us are diving.”

  The inevitable risks of cave diving nonetheless threaten every venture. “You can’t completely prevent silt-outs,” he says, “but if you experience zero visibility, either from silt-out or from light failure, you keep your act together by ensuring that all the divers stay in contact with one another. Even in zero visibility, you can loop your thumb and forefinger around the guideline and follow it back to the entrance.

  “Whole chunks of cave ceilings can collapse without warning,” Meacham adds. “I’ve never been injured by falling rocks, but I’ve followed lines and found house-sized boulders collapsed on top of them. In those cases you need to know what your limits are and what you are comfortable with. Everyone in a team has the right to call the dive off at any time, no questions asked. Everyone’s limits are different, and that is accepted going into the dive. If you are not comfortable being there, we shouldn’t be there, period.

  “There are venomous snakes hidden all through the rain forest, including pit vipers, rattlesnakes, and coral snakes. You have to be as careful as any other traveler in the jungle to stay on the lookout for them. We had a fer de lance in the water once just prior to a dive. As we were clearing brush around the cenote we must have spooked it into the water. Our Maya crew jumped into action and dispatched it pretty quickly.

  “In some of the caves, especially near the coast where fresh water pumps into the Caribbean, the flow is so strong it’s hard to fight your way into a new passage. On rare occasions, particularly in the coastal caves, there can be a flow reversal that reduces visibility and complicates our exit.

  “Cave diving doesn’t have to be a daredevil sport. The keys to safety are training, gear, and judgment. We are risk managers, not risk-takers.”

  In a kind of manifesto he wrote to answer cave diving’s critics, Meacham vividly evokes the challenge of the endeavor.

  We take our life support in with us and we are, for all intents and purposes, measuring time in breaths, not minutes. We are on constant alert, monitoring our gas supply and switching through tanks and scooters as the dive progresses, monitoring our depth, monitoring our decompression obligations, assessing our environment and the other members of our team and ourselves for the slightest sign of anything out of the ordinary.

  On top of that, we are task-loaded to the hilt, constantly equalizing our air spaces to compensate for changes in pressure, holding a light in one hand, adjusting our buoyancy, running the exploration line, surveying, observing, running a scooter . . . all with, in some cases, hundreds of pounds of gear strapped to us.

  In the same manifesto, Meacham makes an eloquent case for exploring the cenotes as one of the last and most rewarding arenas of geographical discovery left on earth.

  Certainly, it is the physical beauty of the caves that is the prim
e attraction. I can say that I am an explorer in the twenty-first century and that is a kick. Caving presents human beings with some of the last frontiers of physical exploration on the planet. And here in Quintana Roo, we are not just planting a flag on some lone mountaintop and then flying home. We live here, we are part of our community. . . . Mountaineers can see the mountain they have to climb, study it, and prepare accordingly. When cave divers pitch into an unexplored cenote, they have no idea what they are going to find at the bottom.

  As they probe the underwater mazes beneath the karst bedrock of the Yucatán, Meacham and his companions demonstrate the mind-boggling complexity of those networks of tunnels and slots and amphitheaters. One of those networks, called Sistema Ox Bel Ha, has been pushed to a total of 167.9 miles of surveyed passageway, with no end in sight. At the moment, the system stands as the longest known underwater cave in the world. No fewer than 143 different cenotes offer entryways from the surface to this aqueous fairyland.

  The embarrassment of riches that stirs the blood of Meacham’s clan is bodied forth in the dazzling variety of complexes so far discovered. In Quintana Roo alone, eighty-two underwater systems have so far been partially explored, most of them not far from the tourist destination of Tulum. As with dry caves such as Lechuguilla, the most unpromising entrance pits can give way to majestic hidden domains. Ox Bel Ha, Mayan for “Three Paths of Water,” was first explored only in 1998, in large part because the most accessible cenotes leading to the system were, in the words of one explorer, “small, complex, and unattractive tunnels.”

  In 2014, a team of divers made an astounding discovery in a pit called Hoyo Negro, part of the Sistema Sac Actun, a close rival to Ox Bel Ha for the longest known underwater cave. More than 130 feet down inside the cenote—a depth at which no sunlight relieves the darkness, and from which staged decompression must be practiced on the way back to the surface—the team found a collection of odd-looking bones. Many of them belonged to megafauna that are now extinct, including saber-toothed tigers, giant ground sloths, and a gomphothere, a relative of the mastodon dated to around 40,000 BP. The pièce de résistance of the dive, however, was the nearly complete skeleton of a human. Before collecting any of the remains, the team carefully documented their find in situ.

  Subsequent analysis proved that the skeleton was that of a woman fifteen or sixteen years old who had stood only four feet ten inches tall. A sophisticated meld of carbon dating with uranium–thorium dating established the age of the skeleton at between 12,000 and 13,000 BP. The Hoyo Negro woman, nicknamed Naia by the team, emerges as one of the oldest skeletons discovered in the Americas, as well as the most complete skeleton yet found in the New World of any human older than 12,000 BP. DNA found in one of Naia’s wisdom teeth identifies her as belonging to a haplogroup of humans of Asian origin found only in the Americas—further corroboration of the old hypothesis of a Bering land bridge exodus followed by rapid dispersal southward across North and South America.

  Divers and researchers think the pit was once a dry cave that functioned as a natural trap into which animals blundered over the millennia. Naia may have been a young woman out looking for water, who was either killed by a fall or could not escape from the chasm into which she had tumbled.

  The fundamental allure of cave diving for Meacham centers on his passion for exploration. “There is the thrill of going where no one has ever before gone,” he says, “but more than anything I’m motivated by the idea that we are slowly putting the pieces of an enormous jigsaw puzzle together, one for which we do not have the box-top picture. Every hike in the jungle and every dive into a cenote helps put that picture more into focus.” In the past, to discover new cenotes, Meacham and his partners simply hiked through the rain forest looking for pools of water. By now, however, he has developed a high-tech procedure involving analysis of satellite photos. As he explains, “We operate from the simple hypothesis that during the annual dry season the vegetation surrounding cenotes will be more lush than the surrounding jungle, because it has year-round access to water. By taking satellite imagery from 1984, one of the driest dry seasons on record, and applying a filter that accentuates moisture content in vegetation, we’ve been able to find a number of new cenote entrances in the Ox Bel Ha system.”

  Once such a prospect has been identified, divers must hike overland, guided by GPS readings, to the telltale spot. More often than not, the cenote proves to be little more than a dead-end pool. But there’s always the tantalizing promise of finding the portal to another Ox Bel Ha. So far Meacham and his colleagues have been able to handle the complicated logistics of transporting tons of equipment to remote base camps. Using vehicles, horses, and manpower, they have leapfrogged their way from cenote to cenote in an elaborate game of connecting the dots. But fresh challenges remain—among them, exploring cenotes too remote to reach overland. “We might fly in to these sites and hope there’s a clearing big enough to land a chopper,” he muses. “If not, we might even rappel out of the helicopter to reach the ground.”

  For the small bands of diehards who have devoted their careers to exploring the cenotes, being “an explorer in the twenty-first century” is all a man (or woman) could ask for. “Cave diving in the Yucatán,” Meacham claims, “is the most sublime, surreal, and calming thing that I have ever experienced in a life that is rich with experiences.”

  The whims of geology are all but unfathomable. According to John Middleton and Tony Waltham, in The Underground Atlas (1986), some 15 percent of the contiguous United States (excluding Alaska and Hawaii) is underbedded by the kind of rock that lends itself to caves: limestone, dolomite, marble, and gypsum. On September 9, 1972, cavers made the crawl that connected two massive systems in Kentucky, Mammoth and Flint Ridge. Forty-five years later, Mammoth–Flint remains by far the longest known cave complex in the world, with some 405 miles of explored passages. The second-longest system barely adds up to half the extent of this gargantuan labyrinth.

  Yet, as detailed above, the U.S. is strangely deficient in really deep caves, where vertical relief is measured by the gulf between the highest and lowest nooks and crannies of a grotto that humans have physically explored. Montana’s Tears of the Turtle, the current record-holder, weighs in at 1,629 feet.

  The country of France covers less than one-twelfth the area of the contiguous U.S. (248,000 square miles versus 3,119,000), yet France abounds in wild limestone landscapes that challenge the vertical skills and endurance of the best cavers in the world. One of them is the Gouffre Berger, in the Vercors region near Grenoble. (“Gouffre” is a quaint borrowing from the Greek kolpos, meaning “gulf, pit, abyss,” but also a mother’s breast.) Discovered in 1953, within only three years the Berger was pushed to a depth that won it the distinction of the first cave plumbed anywhere in the world whose depth exceeded 1,000 meters, or 3,281 feet. It’s also a remarkably beautiful piece of underground real estate. Middleton and Waltham claim that the Berger is “commonly regarded as the finest caving trip in the world.”

  I’ve climbed and hiked in the Vercors, and one day, curious about the demimonde beneath its grassy surface, I sauntered up to the entrance of another cave that ranked not far behind the Berger. I was stunned by how nondescript that dark hole at the foot of a soaring cliff looked. By the beam of my headlamp, I saw that I could worm my way along a downward-sloping ramp for 20 or 30 feet, and I breathed the dank air pouring out from the depths. But the notion that a whole world of tunnels, tubes, chimneys, and rooms beckoned beyond that gloomy portal was hard to fathom.

  In the 1960s, an ambitious club of cavers based in Lyon started prowling across another pristine massif, that of the Haute Provence some 80 miles northeast of Grenoble. In the commune of Samoëns, not far from the chic ski resort of Morzine, these adventurers pushed through dense thickets of trees and bushes looking for holes in the ground breathing wind. In 1964 they stumbled upon an entrance that they soon realized led to a massive underground maze. They named their cavern the Gouffre Jean-Bern
ard. The club appropriated the first names of two good friends who only the year before had died in a nightmarish scenario, one that all cavers dread. Underground, you have no way of knowing what the weather is doing on the surface. That day, in another cave called the Foussoubie, in the Ardèche, Jean Dupont and Bernard Raffy were oblivious to a storm that drenched the hills above. Their fate arrived in the form of a flash flood surging down the passageways of the Foussoubie. They were swept away by a tide so powerful that their comrades never found their bodies.

  By 1981, Jean-Bernard had been pushed to 4,740 feet below the surface, making it the deepest known cave in the world. Ten years earlier, a shepherd tending his sheep on a tangled hillside not far from Jean-Bernard discovered another strange hole in the ground. For decades ardent cavers probed this system’s complexities, naming the new grotto the Gouffre Mirolda, as they incorporated the first letters of the names of three more companions (Michel, Roland, and Daniel) who had likewise perished in an underground flash flood, this time in the Vercors. In January 2003, explorers extended Mirolda to a remarkable 5,684 feet, elevating it to number one status worldwide. It was a distinction the French cave would retain for a mere eighteen months.