Phong Nha-Ke Bang National Park

A UNESCO World Heritage Site containing the world's largest cave system -- including Son Doong, discovered in 2009, which is large enough to contain a 40-story building and has its own localized weather systems with clouds forming inside its 200-metre-high chambers. The park covers 885 square kilometres of karst mountains in Quang Binh province and contains 400 cave systems, of which only 20 have been explored, making it one of the last great geographical frontiers accessible to tourists.