National Geographic has published a remarkable photographic essay about the caves of the Phong Nha-Ke Bang National Park in Vietnam. I can't reproduce many of them due to US copyright law, but under 'fair use' provisions, here are three of them, reduced in size, just to whet your appetite.
A half-mile-long block of 40-story buildings could fit inside this stretch.
There are many more photographs at the link, in larger sizes, too. Very interesting and highly recommended viewing. This national park goes on my list of places I want to see before I die!