Phong Nha-Kẻ Bàng National Park is a protected area in north-central Vietnam 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 is big enough for boats. The area’s rich wildlife includes tigers, black bears and elephants.