Departure from Hue City. Drive to Quang Binh province; stop at Hien Luong Bridge spanning Ben Hai River, which served as the demarcation line between North and South of Vietnam during the Wartime. Visit Phong Nha cavern by boat, the world's longest underground river, entering this cave is like venturing into the mouth of a giant beast, all seems mysterious since the cave rings with strange noise. Locals say it's music from a banquet hosted by the mountain God, but the acoustic tricks actually echoes, which bounce off the cave's limestone cliffs. Back to Hue. The end.