Phong Nha Cave, located in Phong Nha–Kẻ Bàng National Park in Quảng Bình Province, Vietnam, is a river cave known for its limestone formations and underground waterways. Visitors can take a boat along the Son River into the cave, view intricate formations of stalactites and stalagmites, walk through a dry cave section, and experience kayaking along parts of the underground river. The cave is notable for its long subterranean river, extensive chamber system, and distinctive formations, and forms part of a UNESCO World Heritage Site recognised for its geological features and biodiversity within the surrounding national park.