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.