Located in central Vietnam's rugged Phong Nha-Ke Bang National Park, Hang Son Doong might be the world's largest subterranean cavern. Expedition members enter through Hang En, a mile-long portal that tunnels into the lost world, hidden beneath a ring of mountains. The cave was carved out by the Rao Thuong River, which dwindles to a series of ponds during the dry months of April, May and October. At 2.5 miles long and 300ft wide, much of Hang Son Doong's colossal caverns are still being explored. (Carsten Peter/National Geographic Stock)
Hang Son Doong is 300ft wide and nearly 800ft tall -- room enough for an entire New York City block of 40-storey buildings. There are longer caves (the Mammoth Cave system in the United States) and deeper caves (Krubera-Voronja, the "crow's cave", in the western Caucasus Mountains of Georgia), but none compare to the overall size of this enormous subterranean passage.
A second skylight in Hang Son Doong, caused by a roof collapse long ago, reveals a jungle of hundred-foot-tall trees, lianas and burning nettles. An explorer climbs to the surface, while hikers struggle through the dense vegetation below.
A giant cave column swagged in flowstone towers over explorers swimming through the depths of Hang Ken, one of 20 new caves discovered in 2010. This cave, along with Hang Son Doong, is part of a network of more than 150 caves, many still not surveyed, in the Annamite Mountains near the Laos border.