Pollution is threatening the recently mapped Sac Actun cave system in the Yucatan Peninsula, a vast underground network that experts in Mexico say could be the most important underwater archaeological site in the world.

Subaquatic archaeologist Guillermo de Anda said the cave system's historical span is likely unrivaled.

Some of the oldest human remains on the continent have been found there, dating back more than 12,000 years, and now-extinct animal remains push the horizon back to 15,000 years.

He said researchers found a human skull that was already covered in rainwater limestone deposits long before the cave system flooded around 9,000 years ago.

De Anda said over 120 sites with Maya-era pottery and bones in the caves suggest water levels may have briefly dropped in the 216-mile (347-kilometer) -long system during a drought about 1,000 A.D. And some artifacts have been found dating to the 1847-1901 Maya uprising known as the War of the Castes.