Phong Nha-Kẻ Bàng National Park

A UNESCO World Heritage Site in Quảng Bình Province, this national park shelters one of the world's most extensive karst cave systems, including Son Doong — the largest known cave passage on Earth by volume. The park's rivers flow underground for several kilometres before resurging into jungle-lined valleys, and the surrounding limestone mountains are blanketed in primary tropical forest. Boat tours penetrate Phong Nha Cave along a subterranean river, passing vast chambers hung with stalactites.