Located near the Laos–Vietnam border, Hang Sơn Đoòng has an internal, fast-flowing subterranean river and the largest cross-section of any cave, worldwide,[2][3] believed to be twice that of the next-largest passage. It is the largest known cave passage in the world by volume.
Its name, Hang Sơn Đoòng, is translated from Vietnamese as "cave of the mountain behind Đoòng".[4] Đoòng is the name of a Vân Kiềuvillage.[5]
As a solutional cave, it was formed in soluble limestone[6] and is believed to be between 2 and 5 million years old