Deep within Vietnam's forests there's a cave - it's an underground kingdom no one's ever heard of. Back in 1991, a farmer was wandering around an unexplored cave located in the Phon Nha-ke Bag National Park in Vietnam. Suddenly, right before he could go any deeper, and gush of water came from one of the entrances. Frightened, the farmer decided to leave. Locals in the area named the cave Son Doong, which means mountain river cave. 18 years later, in 2009, Howard Limbert and a group of British scientists decided to locate and explore the cave. When they stepped inside, they were in awe of what they found. Being over 3 miles long and 492 feet wide, Son Doong is the largest cave in the world. This isn’t a cave, it’s a whole mini world.