Sơn Đoòng Cave (hang Sơn Đoòng, "Mountain River cave" in Vietnamese) 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 limestone.
According to the Limberts, the cave is five times larger than nearby caves like Phong Nha Cave and Hang En, the third largest cave in the world 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 runs for approximately 9 kilometres (5.6 mi) and is punctuated by 2 large dolines, which are areas where the ceiling of the cave have collapsed. The dolines allow sunlight to enter sections of the cave and 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.
In early August 2013, the first tourist group explored the cave on a guided tour at a cost of US$3,000 each. Permits are required to access the cave and are made available on a limited basis. Only 500 permits were issued for the 2015 season, which runs from February to August. After August, heavy rains cause river levels to rise and make the cave largely inaccessible.