In a pitch black, 140-foot-deep underwater cave, three divers make a stunning 13,000-year-old discovery: the oldest complete human skeleton ever found in the Americas. In this video, see the ancient remains, venture through the remarkable deep-water chamber, and see how a skeleton belonging to a teenage girl from the last ice age lead scientists to a major revelation about the earliest Americans. The monumental finding has roots in 2007, when cartographer and National Geographic grantee Alberto Nava and his two friends, Alejandro Alvarez and Franco Attolini, were mapping 100,000 feet of underwater cave passages in Mexico’s Yucatán Peninsula. Expecting to see mostly tunnels, the team was shocked when they popped into an unknown 200-foot-wide abyss, littered with the bones of extinct creatures like saber-toothed cats, giant ground sloths, and gomphotheres (elephantlike animals related to mastodons). "This was the discovery of a lifetime. It was mesmerizing," Nava recalls. "We really didn't know what to do. We were sitting there laughing. Our regulators were coming out of our mouths from laughing so much." The overjoyed team would name the giant void Hoyo Negro, which translates to Black Hole. While Hoyo Negro was an amazing finding on its own, it's home to an even more exceptional discovery, many millennia in the making. While exploring the cave, Alvarez found a skull, "and all of a sudden," as Nava recounts, "we discover the most complete, and the oldest human remains ever found in the Americas." Led by Mexico's National Institute of Anthropology and History and supported by the National Geographic Society, Nava and an international group of scientists have spent years working to protect, recover, and study the skeleton. "We have an intact cranium. We have all her teeth. We have most of her vertebra, most of her ribs, all her limbs. We have the pelvis, and we have the pubic bone. It's just amazing," Nava says in wonderment. "I feel proud of the discovery and we're all happy that we did it, but at the same time it's become this responsibility of seeing the site protected, seeing the research and the science done so that we can actually unravel all the secrets of Hoyo Negro," Nava says. "We're the first ones to interact with whatever is left of this human that lived during the last ice age." That human was a teenage girl who lived about 13,000 years ago, before the cave was underwater. The Hoyo Negro team has affectionately dubbed her "Naia," after the water nymphs in ancient Greek mythology. "One of the researchers said, 'Okay, we got to give it a name,'" Nava remembers. "Because it becomes a different relationship. Now you're not talking about an 'it,' you're not talking about bone, you're talking about a person." And Nava thinks about the human element of the discovery frequently. "For me, when we go in these caves, we think that we're doing this great exploration, but in reality the original explorers of these tunnels were Naia and her people. We're just following them 13,000 years later. I just imagine the conditions they were living in at that time. This is a deserted area and they're going in there and having to find water and food and shelter and fight animals," Nava says, wondering just how Naia found her final resting place in Hoyo Negro. "Hoyo Negro and Naia have become an integral part of my life, part of my story." Turns out, Naia is part of a lot of Americans' stories, or at least their histories. Scientists were able to extract ancient, intact DNA from one of Naia's teeth, determining her DNA matches that of modern Native Americans. This suggests that the earliest Americans migrated from Siberia via a now nonexistent land bridge, and any physical differences among populations are due to natural changes that occurred after humans arrived in the Americas, and not from having migrated from multiple locations in Europe and Asia, as some anthropologists previously thought. "The discoveries that we're making inside the cave in the Yucatán Peninsula give us a view of how life used to be and it gives you a sense of how small our existence is compared to what the planet has gone through, to what humans have gone through," says Nava. "For me, Hoyo Negro and Naia are the stars of this project, and we all work to bring their stories light so we understand a little bit better about where do we come from."