Hang Sơn Đoòng cave, Phong Nha-Kẻ Bàng National Park, Vietnam

© Geng Xu/Getty Image

A 'hole' new world. Hang Sơn Đoòng, Phong Nha-Kẻ Bàng National Park, Vietnam

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 3.1 miles 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. It even has its own weather system; clouds form inside the cave due to temperature differences between the cave air and the outside air.

Moreover, a subterranean river links Hang Sơn Đoòng to the nearby Hang Thung cave, suggesting yet 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.