World’s Longest Underground River

World’s Longest Underground River Discovered in Mexico, Divers Say

John Roach
for National Geographic News

March 5, 2007

Divers exploring a maze of underwater caves on Mexico’s Yucatán Peninsula have identified what may be the longest underground river in the world.

The waterway twists and turns for 95 miles (153 kilometers) through the region’s limestone caverns, said British diver Stephen Bogaerts, who made the discovery with German colleague Robbie Schmittner.


In a straight line, the system would span about six miles (ten kilometers) of land. (Related: “Huge Underground ‘Ocean’ Found Beneath Asia” [February 27, 2007].)

Bogaerts and Schmittner spent four years exploring using underwater scooters and specially rigged gas cylinders to find a connection between the Yucatán region’s second and third longest cave systems, known respectively as Sac Actun and Nohoch Nah Chich (Mexico map).

“We expected to have done it by December 2004,” Bogaerts said. “But, unfortunately, we were unable to make the connection in the area we were looking in, so we had to look somewhere else.”

The team scoured the passages, marking each new twist and turn with carefully labeled rope.

On January 23 the pair headed toward the final connection from opposite sides and used an unopened bottle of champagne to make the final tie-off between the two systems.

“It’s a little bit like planting a flag on the moon or the top of [Mt.] Everest,” Bogaerts said.

Explorer’s Paradise

Gene Melton is chair of the Lake City, Florida-based National Speleological Society’s Cave Diving Section. He said the connection caps 20 years of exploration and mapping in the Yucatán’s underground labyrinth.

“[Bogaerts and Schmittner] saw the trending of certain passages going together, and they started making a major effort to explore it,” he said.

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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