Maya Artifacts Found In World's Largest Underwater Cave

More than 120 artifact sites such as burnt human bones, ceramics, and wall etchings have been found in the caves, some dating back more than 12,000 years. One human skull covered in rainwater limestone deposits is 9,000 years old, de Anda says.

Archaeologists have been exploring these cave systems for decades, and these latest discoveries are consistent with the human artifacts and megafauna previously found in the Yucatán's watery underworld. The enormous length of the massive Sac Actun system makes these new discoveries particularly remarkable.

"It is very unlikely that there is another site in the world with these characteristics," de Anda says in a statement. "There is an impressive amount of archaeological artifacts inside, and the level of preservation is also impressive."

Even so, experts warn that the Sac Actun cave system is threatened by pollution.