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. How large, you ask? Hang Sơn Đoòng"s main passage is 5 kilometres long, 150 metres wide and 200 metres high. That"s tall enough to fit a 40-storey skyscraper! Inside, there is a 60-metre calcite barrier called the "Great Wall of Vietnam" and stalagmites stretching as tall as 80 metres.