Hang Son Doong cave in Vietnam is the largest cave in the world. In 1991 the cave was discovered by a man named Ho-Khanh but it wasn't until 2009 when British scientists surveyed it that its massive size was realized. The cave is over 6 km long and contains large caverns, a river, jungle, and even clouds. One cavern could fit an entire city block of New York skyscrapers. The cave continues to reveal new ecosystems as collapsed ceilings allow sunlight to stream in.