A caver gazes up at towering formations in Vietnam
's Son Doong cave.
The joint British-Vietnamese team that explored the cave in April 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 overall 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.
Photograph by BARM/Fame Pictures