The 10km-long Chay River is an estuary of Son River with a gentle stream flow, pure water and many species of endemic fishes like Cyprinus centralus, Cyprinus sp, Chela quangbinhensis and Hemibagrus vietnamensis. Chay River hides itself in Phong Nha forests with the Vom Cave system like Thien Duong Cave (Paradise Cave), Me Bong Con Cave and Toi Cave (Dark Cave), etc.
Hang Toi (Dark Cave) was discovered in the years of anti-America resistance war by local people and used as a bomb shelter. This cave is a branch of Phong Nha cave system with 5,558m long and 80m high; the entry of cave is 30m high and 10m wide with heavy forest around the cave mouth. Hang Toi has a primitive beauty with thousands of several spectacular stalagmite and stalactites. This is a shelter of many species of animals like bat and swift, especially, Ha Tinh langur - one of primate species listed in the Vietnam Red Data Book. Especially, Mr. L.Deharveng, a Russian scientist, on January 9th 1995, collected, discovered and proclaimed a specimen of a species of new crab (called Nemoron nomas) in Dark Cave. This specimen is now being kept in the Museum of Zoology, the National University of Singapore.