The gate to Phong Nha cave, Ninh Binh. Besides the grotto and cave systems, Phong Nha has the longest underground river. The Son and Chay are the main rivers in this national park. The Son River flows into the mouth of the Phong Nha cave and keeps on underground, where it is called as the Nam Aki River. It emerges 20 km to the south near Pu-Pha-Dam Mountain.