Phong Nha-Ke Bang

Declared a national park in 2001 over an earlier nature reserve, it protects one of the largest areas of limestone karst on the planet, formed over more than 400 million years. Within it lies Son Doong, found by a local farmer named Hồ Khanh in 1991 but not scientifically explored until 2009, by a team from the British Cave Research Association: it is the largest natural cave in the world by volume, with sections so vast that they hold their own rainforest, climate and internal cloud system, and a 40-storey skyscraper would fit whole inside its main galleries. It has been a UNESCO World Heritage Site since 2003.