In 1991, a Vietnamese logger stumbled upon something big in Vietnam"s Phong Nha-Kẻ Bàng National Park. During a jungle expedition, he found the entrance to a cave with a roaring underground river, which was later named Hang Sơn Đoòng, or "cave of the mountain river." In 2009, the British Cave Research Association led an expedition to explore the site. It was then that they realised something incredible: it was the world"s largest cave.