Sơn Đoòng Cave is a solutional cave in Phong Nha-Kẻ Bàng National Park, Vietnam. As of 2009 it is the biggest known cave 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 limestone.
Sơn Đoòng Cave 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 scientists from the British Cave Research Association, led by Howard and Deb Limbert, conducted a survey in Phong Nha-Kẻ Bàng from 10 to 14 April 2009. Their progress was stopped by a large, 60 metres (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.
According to the Limberts, the cave is five times larger than Phong Nha Cave, a nearby cave previously considered the biggest cave in Vietnam. The biggest chamber of Sơn Đoòng is more than 5 kilometres (3.1 mi) long, 200 metres (660 ft) high and 150 metres (490 ft) wide. With these dimensions, Sơn Đoòng overtook Deer Cave in Malaysia in 2009 to take the title of the world’s largest cave. 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. Text via Wiki Image source National Geographic.