National Speleological Society

The National Speleological Society (NSS) is a non-profit membership organization dedicated to the scientific study, exploration, protection and conservation of caves and karst, and to the promotion of responsible cave exploration and management. Founded in 1941 in the USA, the NSS is the largest organization of its kind in the world.

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Largest cave

Hang Son Đoòng ("Mountain River Cave") is the world's largest single cave passage, based on its overall dimensions. It is around 200 metres (655 feet) high, 150 metres (490 feet) wide and has a length of at least 6.5 kilometres (4 miles). Son Đoòng is located in Phong Nha-Ke Bàng National Park, Bo Trach District, Quang Bình Province, Vietnam.