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.
