The world's longest underground river winds through Vietnam's Phong Nha cave system, the name of which translates as " The wind's fang". Entering this cave is like venturing into the mouth of a giant beast, all the more mysterious since the cave rings with strange noise. Locals say it is music from a banquet hosted by the mountain God, but the acoustic tricks are actually echoes, which bounce off the limestone cliffs of the cave. 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.
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.

From the 14th chamber there may be other corridors leading to similarly large chambers, but this area is more dangerous due to ongoing erosion of the limestone of the cavern.

Some 800 meters into the cave we reach the Shallow Cave, where we step out of the boat to explore a fantastic landscape of sand and rock. Stalactites and stalagmites jut out like strange trees, exciting our imaginations.