Son Doong Cave is a solutional cave in Phong Nha-Kẻ Bàng National Park, Bố Trạch District, Quảng Bình Province, Vietnam. As of 2009 it is the biggest known cave in the world, and is located near the Laos–Vietnam border. Inside is a large, fast-flowing underground river. It was formed in Carboniferous/Permian limestone.