Hang Son Doong is a solutional cave in Phong Nha-Kẻ Bàng National Park, Bố Trạch District, Quảng Bình Province, Vietnam. As of 2009 it has the largest known cave passage cross-section in the world, and is located near the Laos–Vietnam border. Inside is a large, fast-flowing subterranean river. It was formed in Carboniferous/Permian limeston and is believed to be between 2 and 5 million years old
Hang Son Doong 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 calcite wall, which was named the Great Wall of Vietnam. It was traversed in 2010 when the group reached the end of the cave passage. [from wikipedia]
