Son Doong Cave

Sơn Đoòng cave, in Phong Nha-Kẻ Bàng National Park, Bố Trạch District, Quảng Bình Province, Vietnam, is one of the world's largest natural caves.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, as of 2009, 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 variously translated from Vietnamese as 'cave of the mountain river' or 'cave of mountains behind Đoòng'.As a solutional cave, it was formed in soluble limestone and is believed to be between 2 and 5 million years old.

Source: Wikipedia