Experts: Vast underwater archeology site imperiled in Mexico

Mexican experts say the recently mapped Sac Actun cave system "is probably the most important underwater archaeological site in the world," but is threatened by pollution.

Subaquatic archaeologist Guillermo de Anda says researchers found a human skull that was already covered in rainwater limestone deposits before the cave system flooded around 9,000 years ago.

De Anda says 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 about 1,000 A.D.

But de Anda said Monday the cave with the stone-encased skull has high acidity levels, suggesting acidic runoff from a nearby open-air dump could damage skeletal remains.

The world's other great underwater site, the sunken Egyptian city of Alexandria, is also threatened by pollution.