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