The Tien Son Cave in Phong Nha-Ke Bang National Park near Dong Hoi, is located in one of the two of the world's largest karst regions.

This NP has around 300 caves and grottoes, ans is also home to Hang Son Doong, i.e. the ‘Mountain River Cave’, as well as to Hang Thien Duong, i.e. ‘Paradise Cave’.

With a height of 200 meters and a width of 150, Hang Son Doong is believed to be the world's largest known cave passage cross-section, twice the size of that of Malaysia's Deer Cave.

And Hang Thien Duong, a cave with a length of 31 km which was discovered only in 2005 by scientists of the British Caving Research Association, is the longest dry cave in the world. The 980 meter long Tien Son Cave, depicted here, is located at an altitude of ca. 200 meter, up the cliff from Phong Nha Cave (fig.) and is accessible by a stairway that starts from the Son River (fig.) below.