HANOI: Three divers who helped rescue a Thai football team last year have made a fresh discovery in Vietnam where they explored a tunnel that could expand the footprint of the world’s largest cave.

The team was invited to descend into a waterlogged pit in the Son Doong cave in central Vietnam that has never been explored and is believed to connect to nearby chambers.

They were forced back at 77 metres (252 feet) because they did not have enough oxygen to push further, but they think the tunnels could be 120 metres deep.

If the tunnel connects to another cave, it would make Son Doong “easily the largest cave in the world and it would never be overtaken,” British cave expert Howard Limbert, who helped organise the dive, said Tuesday at a press conference announcing the find.

The three divers - Rick Stanton, Jason Mallinson and Chris Jewell - were part of the daring rescue to save 12 Thai footballers and their coach who were trapped in a cave for eighteen days last year.

Stanton - who found the boys on a ledge - said the painstaking task of safely leading the group out of the tunnel alive helped to prepare for the mission in Vietnam.