An archaeological expedition aimed at plumbing the secrets of Chichen Itza’s underworld of the gods has uncovered the well-preserved remains of ancient humans and extinct animals that date back to the last ice age–far more than what the expedition’s members bargained for when they set out to map and explore the sacred network of Mayan caves. The nature of some of the fossils found there also hinted at the occurrence of a "catastrophic event" that embedded some of the bones in the walls of the cave.

Launched in September of 2017 by Mexico’s National Institute of Anthropology and History, the expedition set out to map the vast network of subterranean caves and sinkholes of the Yucatan, called "cenotes", using ground-penetrating radar, and sending diving teams to explore the submerged passageways in person. The mapping effort unveiled 248 cenotes connected through a vast, 347-kilometer (216-mile) network called Sac Actun, home to 200 newly-identified archaeological sites, with approximately 140 of those identified as ancient Mayan.

"It is a very rich system in contexts, depths and beauty. It is undoubtedly a place that is one of the last frontiers in exploration in the world, after the oceans," explains underwater archaeologist Guillermo de Anda. "It is an immense system that goes like a huge labyrinth under the earth, even the road and has a system that flows into the sea."

But alongside the relics of this lost civilization was the unexpected find of human remains that were at least 9,000 years old, accompanied by the even older remains of extinct mammals such as giant sloths, elephants and bears, that date back to the Pleistocene. This painted a new picture of how various human populations migrated into the region, with the post-ice age population eventually being supplanted by the ancestors of today’s indigenous cultures. But the expedition also found evidence that the transition between geological ages wasn’t as smooth as one might expect, hinting at an event violent enough to embed some of the fossils in the cave walls.

"In addition we can see that there was an important step of people during a great period of time. In this space is the Pit cenote, where we find bones embedded in the wall that tell us about a catastrophic event. Likewise, we find skulls where Mayan contexts are mixed with ancient ones," says de Anda. 

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