“Phong Nha-Kẻ Bàng National Park is a national park and UNESCO World Heritage Site in the North Central Coast region of Vietnam, about 500 km south of Hanoi, bordering the Hin Namno Reserve in Laos. Characterized by mountains, tropical forests and underground rivers, it’s known for its ancient limestone karsts and vast network of caves. These include the huge Sơn Đoòng cave and Phong Nha cave, which are the biggest caves in the world”
Source: Phong Nha – Ke Bang National Park Headquarter website and Wikipedia.