Expert scuba divers like Schmittner, originally from a small town near Frankfurt, Germany, and Bogaerts, from Greater London, have learned to control their breathing so they use less than a quarter of the air a novice does. They still take plenty of air with them, though. On this day Schmittner went in with five tanks strapped around his body; Bogaerts, who had to wriggle through some narrow passages, called restrictions, brought three. One of these he stashed in a passage along the way, a second he clipped to his butt, and a third he pushed in front.
When a passage closes in, it is all too easy to agitate the silt on the cave floor below, reducing visibility to nearly zero. Skilled cave divers like Bogaerts and Schmittner move gracefully to maintain visibility going in, but getting out alive is still a technical feat. Maneuvering safely requires deft use of a nylon rope that they tie off every time a cave wall turns. When they are done exploring, they turn around and follow the cord to retrace their way back out. Lose the cord, they know, and they might be lost for good.
Getting a full picture of the cave means painstakingly calculating the water depth and taking a compass bearing at every tie-off, then measuring the distance between each set of points. Along with GPS readings gathered at an entrance, these measurements allow explorers to create detailed cave maps. It is a long process; mapping Sac Actun took Bogaerts and Schmittner five years.
Jim Coke, a diver who was the first to explore Sac Actun in the late 1980s, has created a large and growing library of maps of the Yucatán caves through his Quintana Roo Speleological Survey, which aggregates data gathered by cave divers like Bogaerts and Schmittner. First he translates their data into a series of lines representing distances and angles. Then he adds measurements of the width and curvature of the cave tunnels, finally using specialized computer software to generate beautiful maps of the complex underwater terrain.
“Unless you survey a line, you have not been there, because you don’t know where you are,” Bogaerts says. It is only by constructing a record of what they have seen that cave explorers feel they have seen it at all.
The maps do not just chart the territory; they also help gauge pollution drifting down from human activity on land. An hour and a half south of Cancún, most of the water is still pristine, Schmittner says. But massive development—potentially including a new airport—is planned for the area that Mexican authorities tout as “the Riviera Maya.” Contaminants diffuse much more quickly through water than through limestone, so if the caves are all connected, pollution will spread rapidly. At the moment, sewage from hotels and resorts is generally pumped into deep-injection wells 90 meters (about 300 feet) below ground level, sometimes straight into undersea caves. And solid waste is dumped into unlined pits, where groundwater can carry it into the porous limestone.
Some maps remain under wraps, kept in privately held collections by the explorers themselves because they point the way to archaeological and paleobiological treasures that have been protected by the waters for thousands of years. Explorers of cenotes early in the 20th century found treasure troves of Mayan artifacts in the underwater caverns, but these have now largely been plundered. Still, many human fossils from around 10,000 years ago or more remain. Schmittner describes a human skull he found half-buried in limestone. He left it there, marked only in his memory and private records. He says he and Bogaerts also found two human skeletons; the bones appear to date back about 12,000 years, placing them among the oldest human remains in the Americas. The two divers have discovered mastodons at six sites as well as small horses, giant tapirs, and giant sloths. But the Mexican national archaeology agency has just a single archaeologist trained in cave diving, so it could be many years before science catches up with the finds.
“Everything we do, we pay for out of our own pockets, because we love it,” Bogaerts says. “We’re not trying to do it for anyone except ourselves—just go out in the jungle, find cenotes, jump in, and go exploring.”