Tien Son Grotto is located 400m from the entrance of Phong Nha Grotto and 130m above sea level. It is known as the Dry Grotto or Upper Phong Nha. Upon climbing onto the steps, visitors will reach the entrance of Tien Son Grotto. Deep 1,000m inside the mountain, the grotto looks like a stone castle which is high and well-ventilated with white veins that seem to be gilded with silver and gold. Particularly, visitors to Tien Son grotto can enjoy acoustic resonance from slabs of stones and stone columns. When you knock at the stones, they produce unusual sounds that seem to resound from the earth's womb. Tien Son Grotto is 980 m long. Approximately 400m from the entrance is a 10m-deep abyss, and that is also the distance visitors are allowed to go in.

According to the studies, Tien Son Grotto was formed millions of years ago. It is even older than Phong Nha Grotto. Phong Nha and Tien Son constitute a beautiful picture in the complex of caves and grottoes in Phong Nha-Ke Bang Natural Heritage Site.