Deep beneath the tropical forests of the Yucatán Peninsula lies a vast subterranean domain few people can explore. Accessible by sinkholes known as “cenotes” and potentially stretching across thousands of kilometers underground, these are the world’s most extensive underwater cave systems.
Their tunnels are dark and flooded now, but they were dry at times during the Late Pleistocene, a period approximately 126,000 to 11,700 years ago. Proof that humans and animals once roamed deep within these tunnels rests in fossils and traces of human activity that have been undisturbed for millennia. That we know of them at all is thanks to the work of highly specialized divers and their collaboration with teams of international scientists.
One particular part of these caves made headlines in 2014. Hoyo Negro (or “Black Hole”) is an enormous bell-shaped pit in Sac Actun, the second largest cave system in the Mexican state of Quintana Roo. Numerous fossils were found in its depths, including Naia, one of the three oldest human skeletons from the Americas known to date.
Three divers—Alejandro Alvarez, Franco Attolini, and Alberto Nava Blank—discovered the pit in 2007. Three years later, Mexico’s National Institute of Anthropology and History (INAH) created the Hoyo Negro Underwater Archaeological Project (Proyecto Arqueológico Subacático Hoyo Negro of the Instituto Nacional de Antropología e Historia), a team made up of paleontologists, archaeologists, and divers from Mexico, Canada, and the U.S.
Diving into the unknown
Roberto Chávez Arce has been exploring those tunnels since 2011, when he was invited to be a part of the project. He is a project co-director and diver, and his phenomenal photography within Sac Actun was one of the windows that enabled scientists above ground to view its contents.
He described the marvels of being able to witness that subterranean world firsthand in video interviews with Gizmodo. But entering that realm is dangerous, not least of which because when they first began, Sac Actun’s tunnels were largely unknown, unmapped, and in total darkness underwater.
Alberto Nava retrieving one of the last vertebrae of Naia in Hoyo Negro. © Roberto Chávez Arce
To explore the tunnels leading to and around Hoyo Negro, divers have to carry all of the mechanisms that keep them safe, including breathing gear, spools of lines to prevent them from getting lost, and lights to see where they’re going. This is on top of the cameras and video equipment they use to document everything.
For added protection, Chávez Arce explained, they carry “spares of the spares. We need redundant gear in case [anything] fails” while they’re in the depths of the cave system.
Getting to Hoyo Negro isn’t a quick journey, depending on where divers enter the cave system. Initially, Chávez Arce and his fellow divers—usually in groups of two or three—would enter Sac Actun from a cenote that was 3,000 feet (914 kilometers) from Hoyo Negro. Simply swimming from that entrance to the pit would take a little under an hour, an important detail when one is dependent upon the limited oxygen of diving gear.
But that changed over time. Accessing Hoyo Negro is currently much faster for two reasons. They have since discovered another cenote that is about 250 to 300 feet (76 to 91 meters) from the pit, and the divers are now propelled through the water by motorized scooters that look like torpedoes.
At first, mapping the cave system meant using rudimentary implements, including “a compass and lines and counting distances with a tape measure,” Chávez Arce said. Eventually, however, they transitioned to structure-from-motion (SfM) photogrammetry. He explained this as taking “pictures that overlap,” then feeding those pictures into software, which creates a 3D point cloud. “That,” he noted, “took us a long time,” requiring “many, many hours of diving.” But what it created is an astounding virtual model of Hoyo Negro and parts of Sac Actun, one that brings the underwater world to scientists who wouldn’t be able to access it otherwise.
One of his photos is enough to show the scale of Hoyo Negro. In it, artificial light illuminates the walls of the pit and the jumble of rocks at its base. The two divers, shown swimming midway to its ceiling, are dwarfed by its dimensions. At its rim, Hoyo Negro has a diameter of over 120 feet (32 meters); the bottom of the pit expands to a diameter of over 203 feet (67 meters); and it is almost 200 feet (60 meters) deep. It’s huge.
When the caves were dry, accessible and possibly inviting
Studying sediment cores and ancient bat guano deposits and seeds helped the team determine that the water level in this chamber, and in the three tunnels that connect to it, fluctuated over time. They found that water reached the bottom of Hoyo Negro at least 9,850 years ago. Water in the cave system continued to rise in step with sea level increases, so that by approximately 8,100 years ago, Hoyo Negro and its upper passages were flooded, and by about 6,000 years ago, the entire cave system was underwater.
This is significant because it indicates when people and animals could access the cave, as well as providing a clue as to why they would do so. Notably, the Pleistocene ecosystem of Quintana Roo was a lot different than what it is now. Instead of a lush jungle, the area was more of a savanna. The drinking water available within the caves would have been a powerful draw. So, too, the cooler temperatures in the heat.
But a dry cave still has dangers, as demonstrated by the fossils that remain within it. Falling 10 stories into this pit meant perilous injury, if not immediate death, and its high walls prevented escape. The bones of the lone human among the many mammal fossils found at the bottom of Hoyo Negro are a case in point.
The same three divers who discovered Hoyo Negro also found Naia, who was named by team member and diver Susan Bird. Bird was responsible for carefully handling Naia’s remains underwater for measurements before eventually transporting them to safety in Mexico, after signs that outside divers were disturbing the site. Naia is neither the first nor the only human found in these cave systems, but, dated to approximately 12,970 to 12,770 years, she is the most complete human skeleton of the three oldest known to date in the Americas.
Sadly, her short life ended in tragedy. Her adult teeth hadn’t fully developed, and tell-tale signs in her limbs indicate that she was not yet 20. Based on these factors, the team estimates that she died when she was approximately 15 to 17 years old and suffered a broken pelvis that occurred at or around the time she died by falling into the pit.
“She landed right on her pubic bone,” Dr. James Chatters explained in a phone interview with Gizmodo. “That’s what broke on both sides.”
Chatters is another co-director of the project who,