It sounds like the premise to an Indiana Jones movie, but it’s real life: The eerie crevices and taverns of the world’s largest underwater cave system contain the remains of ancient humans and extinct beasts from a bygone age.

A skull that belonged to a person who lived 9,000 years ago is among the bounty of relics found within the flooded caves of San Actun, according to an Instituto Nacional de Antropología e Historia (INAH) statement released Monday. INAH archeologist Guillermo de Anda, who is the director of the Gran Acuifero Maya (GAM) project that is leading the effort to map these caves, told Reuters that San Actun is “the most important underwater archaeological site in the world.”

De Anda and his colleagues announced the record-breaking dimensions of this system, which stretches along 347 kilometers (216 miles) of the Yucatán Peninsula, in January, after divers found that a huge submerged cave network called the Dos Ojos System is connected to San Actun.

These newly linked cave systems contain the bones of extinct Pleistocene animals like giant sloths, short-faced bears, and the gomphotheres, an elephant-like herbivore. It’s possible that these animals fell into structures called “cenotes,” which are water sinkholes that remain common in the Yucatán region to this day. They may have became trapped and drowned, or perhaps some of their bones were simply submerged as sea levels rose at the end of the last Ice Age.

Gomphothere bones. Image: Foto Archivo Gran Acuifero Maya, INAH

Of the nearly 200 archeological sites that have been identified within the San Actun caves, over half belong to the Maya civilization, an immensely successful empire that flourished from 2,000 BCE to 1,000 CE, and has much older roots in Mesoamerica. Some of the human remains may have been deposited after people stumbled into sinkholes, or were forced into them as sacrificial offerings. Likewise, fragments of pottery, artwork, and statues have been found in the caves, which may have been deliberately thrown into the cenotes as part of religious or cultural rites.

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The underwater caves are still far from fully mapped and may be much larger than the estimates established at this point, suggesting there are many more treasures waiting to be uncovered. "There are other caves around Sac Actun that might be connected," Robert Schmittner, GAM's chief of underwater exploration, told TeleSUR.

"We're already close to the next one and they're probably linked,” he added. “If so, the cave system would be longer than 500 kilometers, and it seems to have no end."