Hang Son Doong (Mountain River cave) is the biggest cave in the world. 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 Phong Nha Cave, a nearby cave previously considered the biggest cave in Vietnam. The biggest chamber of Hang Son Doong is more than 5 kilometres (3.1 mi) long, 200 metres (660 ft) high and 150 metres (490 ft) wide. With these dimensions, Hang Son Doong 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.

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