For decades, geologists have known that Vietnam is home to some of the world's most spectacular caves, many of them largely unexplored. Now husband-and-wife cavers have documented perhaps the world's largest: Hang Son Doong, big enough in places to accommodate a New York City block of skyscrapers.
The cave in the Annamite Mountains contains a river and jungle (its name translates to "mountain river cave") and even its own thin clouds, and its end remains out of sight. It's part of a network of about 150 caves in central Vietnam near the Laotian border.
Howard and Deb Limbert of England led the first expedition to enter Hang Son Doong in 2009, but they were stopped a couple of miles in by a huge calcite wall. The team returned recently to climb the wall, take measurements and try to find the cavern's end. Photographer Carsten Peter was there to capture the incredible images of their journey.
Many more photographs taken in Hang San Doong and other newly explored caves have been published in the January issue of National Geographic and on its website, where you can view larger images. The site also has an interactive graphic of the river cave's path
Other popular stories on Yahoo!:
• Massive bird and fish deaths in Arkansas leave experts befuddled
• Lotto winner nearly loses millions by tossing tickets
• Video: Tennis legends Nadal and Federer duel on water