The Son River (fig.) flowing into the mouth of Phong Nha Cave, in Phong Nha-Ke Bang National Park near Dong Hoi, one of the world's two largest karst regions, with ca. three hundred caves and grottoes (fig.).

About one km from Phong Nha Cave, at an altitude of ca. 200 meter up this cliff, is the entry of Tien Son Cave (fig.).