The team was invited to descend into a waterlogged pit in the Sơn Đoòng cave in central Vietnam that had never been explored. It's believed to be connected to nearby chambers.
They were forced back at 77 meters because they didn't have enough oxygen to push further, but they think the tunnels could be 120 meters deep.
If the tunnel connects to another cave, it would make Sơn Đoòng “easily the largest cave in the world and it would never be overtaken,” according to British cave expert Howard Limbert, who helped organize the dive.
The three divers — Rick Stanton, Jason Mallinson and Chris Jewell — were part of the daring rescue to save 12 Thai soccer players 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.
“Our planning and preparation is without parallel,” he said.
The team plans to return to Vietnam next year to try to link the tunnel to another cave near Son Doong, which is so big that it has its own ecosystem and weather patterns.
The cave in central Quang Binh province was first found by a local forager in 1991, but was not re-discovered for another 19 years because its entrance was hidden by thick surrounding jungle.
Only 30 percent of Vietnam’s Phong Nha national park — where Son Doong and a network of adjacent caves are located — has so far been explored.
Son Doong is the world’s largest cave by volume, big enough to house a New York city block — including 40-storey skyscrapers — according to Oxalis, which runs tours into the caves.
