Variously translated as 'cave of the mountain river' or 'cave of mountains, Hang Son Doong is the world's largest natural cave, located in the heart of Phong Nha-Ke Bang National Park in the Quang Binh province of Vietnam. It is the largest known cave passage in the world by volume. Its cross-section is believed to be twice that of the next largest passage, in Deer Cave, Malaysia. It was formed during the Cambrian-Permian geological eras, putting it at a date between 400–450 million years old.
This cave was found by a local man named Ho Khanh in 1991. Only in 2009 did the cave become internationally known after a group of cavers from the British Cave Research Association, conducted a survey in Phong Nha-Ke 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. It is more than 5 kilometres (3.1 mi) long, 200 metres (660 ft) high and 150 metres (490 ft) wide.
As a solutional cave, it was formed in soluble limestone and is believed to be between 2 and 5 million years old. By mid-2019, it became clear that the cave is connected by its underground river with a nearby cave called Hang Thung. This increases the effective volume of the cave by more than 1.6 million cubic metres.
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. The scale of the site’s interior is so massive it could fit an entire New York block inside, including skyscrapers, or have a Boeing 747 comfortably fly through without its wings being in any danger.
The dolines allow sunlight to enter sections of the cave, resulting in the growth of trees as well as other vegetation. 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. You need to be extremely fit and patient, as there is a waiting list of about two years. The government only releases around 300–500 permits yearly.