Going underground, expedition members enter Hang En, a cave tunneled out by the Rao Thuong River. Dwindling to a series of ponds during the dry months, the river can rise almost 300 feet during the flood season, covering the rocks where cavers stand.


A half-mile block of 40-story buildings could fit inside this lit stretch of Hang Son Doong, which may be the world's biggest subterranean passage.


A jungle inside a cave? A roof collapse long ago in Hang Son Doong let in light; plants thickly followed. As "Sweeny" Sewell climbs to the surface, hikers struggle through the wryly named Garden of Edam.
http://ngm.nationalgeographic.com/2011/01/largest-cave/peter-photography
Photography by Carsten Peter