Phong Nha Cave (fig.) in Phong Nha-Ke Bang National Park can be explored by boat.

The Son River (fig.) flows through this cave, which has a length of 7,729 meters, and contains 14 grottoes with a combined length of 44.5 kilometers.

After a while the river becomes subterranean and its name changes into Nam Aki.

At this point the boats exploring the cave have to turn back, whereas the river continues underground for another nearly 14 kilometers.