Shaped like a tube with a roof curved like the hull of a boat, this cave has acoustics properties similar to those of the fingal cave in Scotland, most importantly, all the primitive stone caves were preserved in their original form. Thus, visitors almost feel like they are going on a trip to the center of the earth.

The Son River flows into the mouth of the cave and continues underground, where it is known as the Nam Aki River. It emerges 20 km to the south near Pu-Pha-Dam Mountain.

Phong Nha Caves, also called Troc Caves, lie in the limestone cliffs of Ke Bang in Quang Binh province, 50 km northwest of Dong Hoi town. Like most of the caves in this area, the Phong Nha Caves were shaped by the Chai River . The farther onne gets inside the Phong Nha Caves , the more illusory the stalactites and stalagmites look as they glitter when bright light is shone on them.

The main cave system contains 14 chambers, linked by an underwater river that runs for 1.5km. Secondary corridors branch off in all directions. The Outer Cave and some of the Inner Caves have roofs that tower between 25 and 40 meters above the water level.