The Son Doong Cave in Vietnam is the biggest cave in the world. It's over 5.5 miles long, has a jungle and river, and could fit a 40-story skyscraper within its walls. But nobody knew any of that until four years ago. A local man discovered the cave entrance in 1991, but British cavers were the first to explore it in 2009. Now, tour company Oxalis is running trial tours of the cave and accepting sign-ups for real six-day tours to take place next year. The man who discovered Son Doong didn't go in because the entrance he found had too steep a drop. On next year's tours, visitors will rappel 80 meters to enter Son Doong. See photos and video at Huffington Post.