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 (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.

Inside the cave, there is a 200-foot calcite barrier called the 'Great Wall of Vietnam' and stalagmites stretching as tall as 260 feet. It even has its own weather system; clouds form inside due to temperature differences between the cave air and the outside air. Moreover, an underground river links Hang Sơn Đoòng to the nearby Hang Thung cave, suggesting undiscovered depths. Given its fragile ecosystem, strict regulations are in place to preserve Hang Sơn Đoòng's pristine condition. Tours of the cave are limited to 1,000 visitors per year.