Hang En Cave, Vietnam
Photograph by Carsten Peter, National Geographic
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 (91 meters) during the flood season, covering the rocks where cavers stand.
See more photographs from the January 2011 feature story "Conquering an Infinite Cave."