Tien Son cave is located in Phong Nha-Ke Bang National Park. The cave was discovered by a local inhabitant by accident in 1935. The entry mouth of Tien Son is 1 km from Phong Nha cave, at an altitude of 200 m. This cave is 980 m in length. A 10 m deep hole is situated 400 m from the entry mouth, then a 500 m long underground cave, dangerous for tourists and open to professional expeditionists only.
Like Phong Nha cave, this cave features spectacular stalactites and stalagmites shaped like several fairy-tales. Stalactite and stalagmite columns and walls here create strange sounds like that of gong and drum if they are knocked with the hand. According to British cave scientists, Tien Son cave was created tens of million years ago when a water current holed this limestone mount in Kẻ Bàng. Following a series of landforms and movement of rocks, this mass was levered or lowered, blocking the current and creating what is now Tien Sơn cave while the underground river redirected its current to Phong Nha cave. Tien Son and Phong Nha are clearly part of the same cave system although they appear not to be connected at present. Millions of years ago the Son river probably flowed through Tien Son but it now flows lower down the valley.
Tien Son is filled with enormous columns, stalactites and stalagmites. Perhaps its most surprising feature is its many fallen stalagmites.
Visitors can enter 400 m into the cave via a winding pathway amongst the speleothems. It is not possible to visit Tien Son without touching the formations. Therefore care needs to be taken to use the formations for support only when necessary.
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