Article on the cenotes, what's been found, and what the impact of tourism down there is starting to do.
http://news.yahoo.com/s/nm/20060320/...ment_mexico_dc

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"Hefting air tanks, guidelines and waterproof lamps, they have so far mapped 405 miles of channels that form part of a huge subterranean river delta flowing into the Caribbean sea, and they are only just starting."
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"It is proving to be a totally unique environment," said marine biologist Tom Iliffe of Texas A&M University. "We are finding things down there including forms of life that no one had ever guessed existed, and there is a lot more work to be done."
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And the scary part:

Meacham says human sewage is pumped deep underground, and that at least one water system in the Yucatan has been polluted with fecal matter. The impact of 250 tonnes of trash dumped in landfills each day has yet to be evaluated.

The hundreds of tourists who dive and snorkel each day in any of a dozen cenotes and caves open to the public are also unwittingly destroying the ecosystems before they can be properly understood, Iliffe says.

"Fish are following the divers into the caves and they gobble up all the life, and they (the caves) are left biologically sterile," he said.