Son Doong Cave in Vietnam

Sơn Đoòng Cave is in Phong Nha-Kẻ Bàng National Park, Bố Trạch District, Quảng Bình Province, Vietnam.

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 'mountain cave of Đoòng.
Hang Sơn Đoòng was found by a local man named Hồ Khanh in 1991. The whistling sound of wind and roar of a rushing stream in the cave heard through the entrance as well as the steep descent prevented the local people from entering the cave.

Only in 2009 did the cave become internationally known after a group of cavers from the British Cave Research Association, led by Howard Limbert, conducted a survey in Phong Nha-Kẻ Bàng from 10 to 14 April 2009.Their progress was stopped by a large, 60-metre (200 ft) high flowstone-coated wall,which was named the Great Wall of Vietnam. It was traversed in 2010 when the group reached the end of the cave passage.
Formed of Carboniferous/Permian limestone, the main Sơn Đoòng cave passage is the largest known cave passage in the world by volume – 38.4×106 cubic metres (1.36×109 cu ft), according to Howard Limbert. It is more than 5 kilometres (3.1 mi) long, 200 metres (660 ft) high and 150 metres (490 ft) wide. Its cross-section is believed to be twice that of the next largest passage, in Deer Cave, Malaysia.The cave runs for approximately 9 kilometres (5.6 mi) and is punctuated by 2 large dolines, which are areas where the ceiling of the cave has collapsed. The dolines allow sunlight to enter sections of the cave which has resulted in the growth of trees as well as other vegetation.

The cave contains some of the tallest known stalagmites in the world, which are up to 70 m tall. Behind the Great Wall of Vietnam were found cave pearls the size of baseballs, an abnormally large size.

As a solutional cave, it was formed in soluble limestone and is believed to be between 2 and 5 million years old.