Katie Graham was trying to escape the cave she’d been trapped inside with her teammates for the past three days. She held her breath and cautiously swam underwater through the turbid flood that was all but completely filling an underground corridor, ominously dubbed Skeleton Canyon, in Sistema Huautla, the deepest cave in the Western Hemisphere.

Only a few inches of air existed between the water’s surface and the roof of the cave. When she emerged, she did so face first, her head tilted all the way back to put herself in the best breathing position.

With her nose and mouth pressed to the slimy limestone roof, she calmly inhaled and made a deliberate effort to move forward slowly so as not to create any waves that would disrupt the bell jar of air surrounding her face. When the air pocket ran out, Graham used her legs as antennae, probing her feet around the pitch-black sump to feel for the next pocket of buoyancy ahead of her. Upon locating one, she’d dive under, swim forward, and again come up face first, her head tilted back.

Cavers Explore Mexico's Massive Sistema Huautla In Claustrophobia-Inducing Footage

“On the third time, I came up into something really low,” says Graham. “I couldn’t see anything. I thought, 'this situation isn’t OK; I should backtrack back to camp.'”

For the past three days, Katie Graham, Stephen Gladieux, Tiffany Nardico, Elliot Guerra-Blackmor, and Chase Varner had been trapped 2,199 feet (670 meters) underground, at a low camp in Sistema Huautla. Located in Oaxaca, Mexico, Sistema Huautla is one of the deepest, longest, and, some say, greatest caves in the world.

The team had known a light rain was in the forecast prior to entering La Grieta, one of the many distinct caverns connected to the greater Huautla system. However, the team had assumed the portion of La Grieta in which they were traveling would be relatively dry, even in a flood.

On the first evening of their trip, a “high-water event” knocked Graham off her feet and swept her nearly 328 feet (100 meters) downstream. The force of the flood pinned her down by her backpack, and she struggled to stand up in the torrent.

“It took quite a bit of effort to get out of the water,” she says, modestly. “We got back to camp, traumatized. That’s when we knew something big was happening. Our water source was increasing.”

When Caves Play Tricks

Meanwhile, a sixth team member, Fernando Hernandez, was stuck outside the cave. Hernandez was tasked with carrying most of the food for the trip and had planned to arrive hours after the rest of his crew. When he reached Skeleton Canyon, he realized the flood had blocked all passage. His teammates were trapped.

"I tried to look for areas to bypass and shouted to see if I could hear them without any luck," says Hernandez. "I thought I heard them once, but maybe it was just the cave playing a trick."

Hernandez returned to the surface to inform the rest of the expedition about the situation.

For three days, the five cavers rationed a small bag of Clif Bar protein bars and continued to do their work in the farthest depths of La Grieta. They completed several new aid climbs, conducted surveys, and discovered a new passage they dubbed “Powered by Bars,” which led to a chamber with a dome estimated to be 492 feet (150 meters) tall, or perhaps larger.

View Images At about 479 feet (146 meters) high, the "TAG Shaft" is one of largest chambers in Sistema Huautla. TAG stands for "Tennessee/Alabama/Georgia," where several different surface entrances meet. During the 2018 expedition, cavers pushed through a tight crack at the bottom of the shaft in a successful attempt to connect it to Sistema Huautla. To capture this image of the illuminated cave, the photographer overlayed multiple photographs taken with different exposures. Photograph by Joshua Hydeman, National Geographic

“There were a lot of new discoveries,” says Stephen Gladieux. “A lot of excitement down there.”

At five in the morning of the fourth day, Katie Graham made a second effort to escape in order to catch her flight home. She returned to the water-choked Skeleton Canyon and once again found herself traversing the passage with her head tilted back, her nose and lips pressed to the ceiling, drawing from the pancake of air.

The water levels had gone down, but not by much. Still, it was just enough for Graham, considered one of the best cavers in the world, to make it out.

The following day, with the water even lower, the remaining team members took the opportunity to exit through the watery Skeleton Canyon and up thousands and thousands of feet of rope. Eventually, they all crawled out of an utterly indistinct hole in the ground and into the blinding sunlight.

View Images Scott Trescott, from Costa Rica, climbs over falling rocks while working his way through a canyon passage in Sotano de Agua de Carrizo. Photograph by Joshua Hydeman, National Geographic

Original Exploration

You can’t tell what lies beyond a cave’s entrance by looking at it. And that is one of caving’s great allures—exploring what cannot be easily seen from a safe distance or rendered in advance by technology. The thrill lies in the firsthand experience of making the unknown known, going not because “it’s there,” as the old mountaineering expression asserts, but because you ultimately don’t know what’s there, and can’t know until you take that first step into the dreadful abyss.

“Caving is original exploration. You gotta go to know,” says Bill Steele, one of the world’s foremost speleologists, expedition cavers, and co-founder of Proyecto Espeleologico Sistema Huautla (PESH).

Every year since 2013, PESH, an official project of the National Speleological Society and the United States Deep Caving Team, has been exploring, surveying, and studying the Huautla-area caves.

As an organization, PESH has committed to 10 consecutive years of expeditions to Sistema Huautla, always in April, reliably the driest month. This latest expedition concluded the fifth PESH trip—a halfway point for the ambitious project—and it was one of the most successful years yet, despite the uncharacteristically wet conditions.

View Images Sonia Meyer works her way through a canyon passage in Sistema Huautla, the deepest cave in the Western Hemisphere. This year, the system is being explored by two separate expedition teams, PESH (Proyecto Espeleologico Sistema Huautla) and Pena Colorada 2018. Photograph by Joshua Hydeman, National Geographic

Following a “lead” discovered last year, the 2018 PESH expedition made a significant connection between a cave named Sotano de Agua de Carrizo and Sistema Huautla. This connection added 4.3 miles, or seven kilometers, to the system’s length, as well as five new entrances, making one of the longest, deepest caves in the world even longer and more complex.

Sistema Huautla is now known to be 53 miles long and has 25 distinct entrances. With a depth measurement of 5,118 feet (1,560 meters) from its highest known entrance to its lowest reached point, System Huautla is the deepest cave in the Western Hemisphere and the ninth deepest cave in the world.

For reference, most major caves have only one or maybe two entrances—and therefore only one or two routes through the cave system. For example, Veryovkina, in the country of Georgia, is the world’s deepest cave, at 7,231 feet (2,204 meters), but it’s a mere 7.9 miles long and has just one entrance.

“Carlsbad Caverns is known for one very big chamber called the Big Room,” says Steele. “We've got at least 12 of them in this cave area. One of them is twice the size of the Dallas Cowboys’ stadium.”

View Images A member of the expedition travels through a portion of Sistema Huautla that sits behind one of Skeleton Canyon's water-filled passages. Traveling through the canyon requires stepping on thin scalpel-like blades, which can easily break under force. To capture this image of the illuminated cave, the photographer overlayed multiple photographs taken with different exposures. Photograph by Joshua Hydeman, National Geographic

It’s the sheer complexity of Sistema Huautla that justifies its reputation as perhaps the world’s greatest cave. And, despite having been first explored by cavers in 1966, Sistema Huautla remains a frontier of science and adventure.

“After a lifetime of exploration, we have no way to predict how much of it we've explored,” says Steele. “My gut tells me that we've probably only reached two-thirds of it, if that—maybe only half!”

Steele, 69, made his first trip to Sistema Huautla in 1977. This year marks his 25th expedition to the region. He calls Sistema Huautla “the masterpiece of my contribution to speleology.”

A Perfect Storm of Geology

Caves become longer or deeper only through firsthand exploration—a physical connection must be made from one cave to the next. Sistema Huautla is the substructure of the Sierra Mazateca mountains, a jungle-covered range of limestone hiding a mind-boggling underworld of porous karst tunnels, waterfalls, and chambers.

“Leads” are places where one cave might connect to another. A lead could be the sound of water in the distance or the feeling of wind blowing through a faint crack in the wall. It could be an enormous tunnel or a crack so tiny a person could barely squeeze through.

Leads are identified, marked, and later explored by a team using state of the art tools for surveying. Modest excavation tactics may be used to remove mud, sediment, or rocks impeding travel. The goal is to make that connection in order to extend the cave’s depth or length. Ultimately, a more complete picture of the geology begins to take shape.

“Sistema Huautla is just the perfect storm of geology for cave development,” says Steele.

The distinct geology of a cave is the canvas upon which all other areas of science necessarily follow: paleontology, archeology, biology, and more.

“We have made some really interesting biological discoveries because the caves are isolated from other areas and they're very old, so there's a lot of unknown species here,” says Steele. “Around 48 species have been found in these caves that don’t live anywhere else.” A newly discovered tarantula was recently named after Steele.

Two years ago, PESH cavers discovered some Pleistocene megafauna bones believed to be the skull of a giant ground sloth.

“We found a complete skull there last year, and we think that there may be two of them,” says Steele. “We think the entire skeletons may be there.” A team of paleontologists is scheduled to begin working on the site in 2019.

One of the more interesting endemic species is alacran tartarus, a troglobitic scorpion, meaning that it’s cave-adapted: blind and with reduced pigmentation. It can also swim, but not breathe, underwater. The degree to which the scorpion is poisonous is unknown.

New species of scorpions are always of interest to scientists because they could potentially be used to develop a more effective antivenin, which could have a significant impact given that each year around 50,000 people around the world die from scorpion stings. According to Dr. Oscar Franke, the scientist in Mexico City who is working with PESH on this scorpion, the best way to learn about the lethality of the scorpion is to either collect a living sample, or be stung firsthand.

“We’ve been told that if someone gets stung, to keep notes,” says Steele. “But that hasn’t happened yet.”

View Images British caver Martin Hoff, one of several international members of the expedition team, rappels into an entrance of Sistema Huautla. Photograph by Joshua Hydeman, National Geographic

Appeasing the Cave Spirits

Steele was raised in southern Ohio, saw his first cave at age four, and started exploring caves in Kentucky as a Boy Scout, sparking a lifelong passion for subterranean adventures. He went on to become a national-level director of the Boy Scouts in Irving, Texas, where he now lives and is retired.

He has a robust mane of bone-white hair, a scholarly white beard, and glasses. To local Mazatec people, Steele looks like “someone from Mars,” he says. Yet he speaks in a calm, unthreatening demeanor—an asset, perhaps, in communicating with people who are stalwart isolationists and inherently suspicious of foreigners tromping around their lands.

The Mazatec people—an indigenous group with their own language and a regionally dispersed population of around 200,000—have long believed the caves to be entrances to the spiritual underworld. Two years ago, members of the Mazatec community asked the PESH cavers to partake in a ceremony to appease the cave spirits.

“They said, 'you've been coming here for years without doing this, so you got a big catch up to do,'” recalls Steele. He was asked to buy a live turkey from a local market and meet the curandero, or shaman, at the entrance to one of the caves. Here, Steele, having by now “bonded with the turkey,” arrived to a scene in which the curandero was leading prayers and chanting in Mazatecan. Plumes of incense smoldered all around.

View Images Amy Morton, PESH expedition team member, describes the experience of sleeping underground as "a lot of vivid dreaming, a lot of water, a lot of imagining you hear voices in the water...” Photograph by Joshua Hydeman, National Geographic

“I was asked throw the turkey down into a 200-foot pit,” says Steele. “I questioned three times if that was being translated correctly, because I couldn't imagine he really meant kill the turkey—but that's what he meant.”

Afterward Steele says the curandero asked him if he had ever seen any cave spirits.

And though Steele hadn't seen any spirits, he told the curandero that he did feel their presence. "There's been some incidents, where there should've been a fatality but it didn't happen, that seemed miraculous at the time," he says. "Not that there's a fairy godmother watching over you, but there's some things that have happened that make you think."

View Images In a dark passage in Oaxaca, Sonia Meyer sketches in a waterproof notepad and Jesse Houser uses a Disto X to shoot down a pit. The cavers use these techniques to keep track of their findings and collect data to build out the map of the system. Photograph by Joshua Hydeman, National Geographic

At this point, Steele sees one of his primary roles as being an ambassador for caving and cavers—especially to the Mazatec people. This is important not just for the future of caving and science in the region, but also for gaining access to higher caves that could ultimately connect into Sistema Huautla, potentially bumping the cave up in the world rankings of deepest caves.