Jo Tuckman

CAVE DIVERS in Mexico have discovered what they claim is the world's largest submerged cave system effectively an underground river beneath Mexico's Yucatan Peninsula. Stephen Bogaerts and Robbie Schmittner had spent four years exploring whether the Sac Actun system links to other cave networks before they made the final connection that revealed a single system that is 95 miles long.

The two divers entered the system separately on January 23 and worked their way through huge chambers and tiny tunnels to meet up at the connection point they had always believed they would find. Mr. Schmittner was carrying a bottle of champagne, which they left secured to the spot.

"It was like putting a flag up on Everest," said Mr. Bogaerts, who says it took some 500 dives of several hours each to get to that point. "We're still walking on air."

Mr. Bogaerts says that putting Sac Actun at the top of the global table of submerged cave systems is far from the end of the story. He and Mr. Schmittner are working on exploring whether Sac Actun connects to a 58 km-long system called Dos Ojos.

It could yet prove to be the longest cave system of any kind a record held by the 360-mile, dry Mammoth Cave system in Kentucky, United States.

The Yucatan Peninsula particularly the area running south from the resort city of Cancun, on the north-eastern tip, along what is known as the Riviera Maya is full of holes in large part because of the combination of porous limestone bedrock, rainforest cover and relative flatness.

Over the millennia the substantial amount of rain that falls here has meandered its way towards the sea, turning slightly acidic because of the dissolving rock, and carving out an extraordinary underground labyrinth. The caves are entered via open pools known as cenotes.

Guardian Newspapers Limited 2007