Hang Sơn Đoòng, widely called Sơn Đoòng Cave, is in Vietnam. It lies within Phong Nha–Kẻ Bàng National Park in Quảng Trị province, near the Laos–Vietnam border.
Sơn Đoòng is recognized as the world's largest known natural cave passage by volume. Its enormous dimensions include a passage roughly 9 kilometres long, with sections reaching about 150 metres high. An underground river runs through the cave, and collapsed sections of the roof allow sunlight and rain to support vegetation inside.
A local man, Hồ Khanh, discovered the entrance in 1990, but the cave was not fully explored and surveyed until a British-Vietnamese expedition in 2009. Its Vietnamese name translates roughly as “cave of the mountain behind Đoòng,” referring to a nearby Vân Kiều village.
“Largest cave” can be confusing because records may measure length, volume, passage size or total connected passages. Sơn Đoòng’s famous distinction concerns the size of its natural passage, not necessarily every possible cave record.