This undated photo released by Mexico's National Anthropology and History Institute (INAH) shows a diver from the Great Mayan Aquifer project looking at human remains believed to be from the Pleistocene era, in the Sac Actun underwater cave system, where Mayan and Pleistocene bones and cultural artifacts have been found submerged, near Tulum, Mexico. Mexican experts said Monday, Feb. 19, 2018, that the recently mapped Sac Actun cave system "is probably the most important underwater archaeological site in the world," but is threatened by pollution.

(Great Mayan Aquifer Project-INAH via AP)

At a Glance

The more than 200-mile long Sac Actun cave system on the Yucatan Peninsula is rife with ancient artifacts.The massive cave system holds some of the oldesthuman and extinct animal remains in North America.The pristine waterand the treasures it holds is now under threat because of pollution and development.

An underwater cave system inMexico considered by experts to be the "world's most important underwater archaeological site" is under threat from pollution and development.

The more than 200-mile long Sac Actun cave system on the Yucatan Peninsula is rife with ancient pottery and other artifacts dating back to the Mayan civilization and beyond. Subaquatic archeologistGuillermo de Anda told the Associated Press the site is a treasure because it holds some of the oldesthuman and extinct animal remains in North America.

Some of the human remains date back 12,000 years, while some called, and bears, date back toat least15,000 years, National Geographic reported.

There are at least 120 sites within the massive cave system that contain Mayan artifacts. De Anda told AP the Mayans didn't live in the caves but likely explored the caves"during periods of great climate stress, to look for water."

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But the pristine waterand the treasures it holds is now under threat because of pollution and development.

A nearby dump has been releasing acidic runoff into the water, which finds its way into the caves. High acidity levels were detected in one skull encased in stone that dates back some 9,000 years.

People are another threat, de Anda said. Tourists are using sinkhole lakes that serve as entrances to the cave systemto snorkel and swim, while roads that run above the caves, including the major highway in the coastal state ofQuintana Roo, are collapsing into sinkholes that open up into the caves.

In January,researchers from theannounced that Sac Actun had become the world's largest knownunderwater cave system after discovering that two systems on the peninsula were actually connected.