World's Longest Underground River Discovered in Mexico,

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Long a popular retreat for beachgoers, the Yucatán Peninsula has become a favorite destination for cave divers, Melton added.

"Just about any time you go you can nearly always go find a new place to explore," Melton said.

He likens the region to "a huge limestone sponge."

That's because the peninsula is largely made of limestone, a soft and porous rock that is easily eroded by slightly acidic rainwater, which carves out underground passages as it courses toward the Caribbean Sea.

The pathways range from jumbo-jet-size rooms with long stalagmites and stalactites to narrow slits that divers must blindly squeeze through.

The passages are completely flooded with water that stays a constant 76 degrees Fahrenheit (25 degrees Celsius) year-round.

The water itself is layered: A lens of freshwater rests on top of salt water. When fresh rainwater percolates down, the liquid flows out horizontally and is discharged into the ocean.

Divers access the caves through sinkholes called cenotes, which lay scattered throughout the peninsula under the rain forest canopy.

"But the water isn't just flowing through these underground rivers ... 98 percent of the water is actually trapped in the rock," Bogaerts, the diver, said.

Conservation Call

The Yucatán's natural hydraulic system sustained the Maya for centuries and today is the main freshwater source for the region's booming tourism trade.

But the cave diving community is concerned that the rapid pace of development could stress the supply.

"These cave systems are so extensive and so interconnected that if there is a point of pollution in one area then it can quickly get distributed to a very, very wide area," Bogaerts said. (Related: "Under-Ice Lakes in Antarctica Linked by Buried Channels" [April 19, 2006].)

The explorers hope their discoveries will help bring attention to the caves, which suffer the "out of sight, out of mind" problem.

"We still have a great deal more to do," Bogaerts said. "There are other cave systems nearby that we are currently trying to connect into this system, and one of the goals of that is to show everybody how interconnected this [underground river system] is."

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