Vietnam is home to Sơn Đoòng, the world’s largest known natural cave by volume. Located in Phong Nha–Kẻ Bàng National Park near the Laos border, its main passage is more than 5 kilometers long, about 200 meters high and roughly 150 meters wide in places.
A local forest explorer, Hồ Khanh, found the cave entrance in 1990 while searching for agarwood. The cave was later surveyed by a British–Vietnamese expedition in 2009, which established its extraordinary scale. Its name is commonly translated as “mountain river cave,” referring to the underground river that runs through it.
Sơn Đoòng formed in soluble limestone over millions of years. Parts of its roof have collapsed, creating huge dolines where sunlight enters and trees grow inside the cavern. Readers sometimes confuse “largest cave” with “longest cave”: Sơn Đoòng holds the volume and passage-size distinction, while other caves may rank higher for total mapped length.