Ora Cave, Papua New Guinea

Photograph by Stephen Alvarez , National Geographic

The crash of rapids surrounds expedition leader David Gill, left, and Herb Laeger near the entrance of Ora Cave. Hot acidic waters welling up from deep underground, combined with drainage from heavy rains, have carved a maze of tunnels and chambers through New Britain s young limestone.

See pictures from the September 2006 feature story "Raging Danger."