A British diver and his German partner have discovered what they claim is the world's largest submerged cave system - effectively an underground river - beneath Mexico's Yucatán 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."

The diver was speaking by phone from his home in the coastal town of Tulum, a tourist haven beside the Caribbean that in recent years has also become a Mecca for the international cave diving community.

Mr Bogaerts is from south London but has spent much of his life exploring caves around the world. He settled in Mexico eight years ago.

The 42-year-old explorer 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 58km-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.

The Yucatán Peninsula - particularly the area running south from the resort city of Cancún, 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

Mr Bogaerts says divers such as he are the "eyes and hands" of the geologists, chemists, hydrologists, biologists and other scientists who are also fascinated by this natural hydraulic system that sustained Mayan civilisation for centuries and remains the main source of fresh water for the tourism industry.

Mr Bogaerts says one of his motivations is to raise the industry's awareness of just how interconnected, and so also vulnerable, the system is, because if you contaminate one part you risk contaminating the whole thing. "Awareness is growing," the diver said, "but it must keep pace with the extremely rapid pace of development."