In 1991, logger Hồ Khanh stumbled upon something big in what is now Vietnam"s Phong Nha-Kẻ Bàng National Park. During a jungle expedition, he found the entrance to a cave with a roaring underground river. It was later named Hang Sơn Đoòng, or "cave of the mountain river" and in 2009, the British Cave Research Association led an expedition to explore it. They realized something incredible: It was the world"s largest cave. Hang Sơn Đoòng"s main passage is 3.1 miles long, 490 feet wide, and 660 feet high. That"s tall enough to fit a 40-story skyscraper.