Besides the grotto and cave systems, Phong Nha has the longest underground river. The SÆ¡n and Chay are the main rivers in this national park. Most of caves here have been shaped by Son and Chay Rivers. 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. There are spectacular streams, springs and waterfalls in Phong Nha-Kẻ Bà ng, namely: Gió waterfall, Madame Loan waterfall, Má»c stream erupting from a limestone mount range, and Trạ Ang stream.