At 262-by-262 feet (80-by-80 meters) in most places, the Son Doong cave beats out the previous world-record holder, Deer Cave in the Malaysian section of the island of Borneo.
Deer Cave is no less than 300-by-300 feet (91-by-91 meters), but it's only about a mile (1.6 kilometers) long.
By contrast, explorers walked 2.8 miles (4.5 kilometers) into Son Doong, in Phong Nha-Ke Bang National Park, before being blocked by seasonal floodwaters—and they think that the passage is even longer.
In addition, for a couple of miles Son Doong reaches more than 460-by-460 feet (140-by-140 meters), said Adam Spillane, a member of the British Cave Research Association expedition that explored the massive cavern.
Spillane was in the first of two groups to enter the cave. His team followed the passage as far as a 46-foot-high (14-meter-high) wall.
"The second team that went in got flooded out," he said. "We're going back next year to climb that wall and explore the cave further."
A local farmer, who had found the entrance to the Son Doong cave several years ago, led the joint British-Vietnamese expedition team to the cavern in April.
The team found an underground river running through the first 1.6 miles (2.5 kilometers) of the limestone cavern, as well as giant stalagmites more than 230 feet (70 meters) high.
The explorers surveyed Son Doong's size using laser-based measuring devices.
Such modern technology allows caves to be measured to the nearest millimeter, said Andy Eavis, president of the International Union of Speleology, the world caving authority, based in France.
"With these laser-measuring devices, the cave sizes are dead accurate," he said. "It tends to make the caves smaller, because years ago we were estimating, and we tended to overestimate."
Eavis, who wasn't involved in the survey, agreed that the new findings confirm Son Doong's record status—despite the fact that he had discovered Borneo's now demoted Deer Cave.
"This one in Vietnam is bigger," Eavis conceded.
However the British caver can still claim the discovery of the world's largest cave chamber, Sarawak Chamber, also in Borneo.
"That is so large it may not actually be beaten," he said. "It's three times the size of Wembley Stadium" in London.
Noisy and Intimidating
Son Doong had somehow escaped detection during previous British caving expeditions to the region, which is rich in limestone grottos.
"The terrain in that area of Vietnam is very difficult," said expedition team member Spillane.
"The cave is very far out of the way. It's totally covered in jungle, and you can't see anything on Google Earth," he added, referring to the free 3-D globe software.
"You've got to be very close to the cave to find it," Spillane said. "Certainly, on previous expeditions, people have passed within a few hundred meters of the entrance without finding it."
The team was told that local people had known of the cave but were too scared to delve inside.
"It has a very loud draft and you can hear the river from the cave entrance, so it is very noisy and intimidating," Spillane said.
Bigger Caves Waiting?
Of more concern to the caving team were the poisonous centipedes that live in Son Doong.
The explorers also spotted monkeys entering through the roof of the cave to feed on snails, according to Spillane.
"There are a couple of skylights about 300 meters [985 feet] above," he said. "The monkeys are obviously able to climb in and out."
A biologist will accompany the team on its return visit next year to survey the cave's subterranean wildlife.
Eavis, of the International Union of Speleology, added that there are almost certainly bigger cave passages awaiting discovery around the world.
"That's the fantastic thing about caving," he said.
Satellite images hint, for example, that caves even larger than Son Doong lie deep in the Amazon rain forest, he said.
Source: National Geographic / Photos: Barcroft Media