Phong Nha–Kẻ Bàng National Park is located in the territories of communes (Vietnamese: xã): Tan Trach, Thuong Trach, Phuc Trach, Xuan Trach and Son Trach of Bố Trạch District and a small part of Minh Hóa District in the center of Quảng Bình Province, around 40 km north-west of the provincial capital city of Đồng Hới. The park is bordered by the Hin Namno karst area of Khammouane Province of Laos in the west. Road distances are about 500 km south of the capital Hanoi and 260 km north of the port city of Đà Nẵng. The geographical coordinates are.
The park is situated around 30 km west of the South China Sea and National Route 1A, near Hồ Chí Minh Highway and 28 km west of the Hanoi–Saigon Railway, and is accessible by road or waterway through an estuary in the South China Sea. There is a small airport near the park accessible by helicopter or small aircraft (Khe Gat Airbase), an airbase used by the North Vietnamese Air Force during the Vietnam War, notably in the Battle of Đồng Hới.
Phong Nha-Kẻ Bàng National Park is one of the world’s two largest limestone regions. In comparison with 41 other world heritage sites which have karsts, Phong Nha has dissimilar geomorphic, geologic and biotic conditions. The karsts of Phong Nha can be traced back to Palaeozoic era, 400 million years ago. This makes Phong Nha the oldest major karst in Asia. If the Hin Namno, bordering Phong Nha on the west (in Laotian territory) was to be combined with the national park in a continuous reserve, the combined reserve would be the largest surviving karst forest in southeastern Asia (317,754 ha).
In general, there are two groups of landforms in the Phong Nha-Kẻ Bàng area, namely non-karstic and karstic landforms. Non-karstic landforms includes three types: The middle and low dome-block mountains developed in intrusive magmatic massifs; The middle denudation-structural mountain belts developed in terrigenous rocks of Cretaceous age; and The low block-denudational mountain belts developed in other terrigenous rocks. Karstic landforms in this area are of typical tropical karst which are divided into two groups of forms: The karstic forms on the surface including cone and tower karst, karrens, valleys and dolines, border polje, etc.; The underground karst consisting of caves.
In comparison with three other national parks listed in UNESCO’s World Heritage Sites in Southeast Asia, namely Gunung Mulu National Park in Malaysia, Puerto Princesa Subterranean River National Park in Palawan of the Philippines and Lorentz National Park in West Irian of Indonesia and some other karst regions in Thailand, China, Papua New Guinea, karst in Phong Nha-Kẻ Bàng is older, has more complicated geological structure; diverse and complex underground rivers.
This cave, from which the name to the whole system and the park is derived, is famous for its rock formations which have been given names such as the “Lion”, the “Fairy Caves”, the “Royal Court”, and the “Buddha”. This cave is 7729 m long, contains 14 grottos, with a 13,969 m-long underground river. The scientists have surveyed 44.5 km of grottos in this cave so far, but tourists can only penetrate to a distance of 1500 m.
Phong Nha Cave, like most of the caves in this area, has been continuously shaped by the Chay River. As one gets farther into the cave, the more illusory the stalactites and stalagmites look as they glitter when bright light is shone on them. The Son River flows into the mouth of the cave and keeps flowing underground, where it is referred to as the Nam Aki River, then this river emerges at a site 20 km to the south near Pu Pha Dam Mountain. The main Phong Nha cave includes 14 chambers, connected by an underwater river that runs for 1.5 km. Secondary corridors branch off in all directions. The Outer Cave and some of the Inner Caves have roofs that tower between 25 and 40 meters above the water level. From the 14th chamber there may be other corridors leading to similarly large chambers, but this area proves more dangerous for explorers because of the ongoing erosion of the limestone of the cavern. The Shallow Cave is located 800 meters from the cave mouth, where there is a spectacular landscape of sand and rock. Stalactites and stalagmites jut out like strange trees, exciting visitor’s imaginations.
Notable caves and grottoes in the Phong Nha Cave system include:
- Dark Grotto (Hang Tối): located upstream of Son River running to the branch of Chay River on the left. This cave is 5,258 m long and with a height of 83 m. The entry is high and wide with heavy forest around the cave mouth
- E Grotto (Hang E): a 736 m-long grotto accessible by the Hồ Chí Minh trail
- Cha An Grotto (Hang Chà An): a 667 m-long grotto, with the width of 15 meters
- Thung Grotto (Hang Thung): This grotto has an underground river of 3351 m, with the height in some part exceed 133 . This river receives water resources from Rao Thuong
- En Grotto (Hang Én): a 1645 m long, 78.6 m high grotto
- Khe Tien Grotto (Hang Khe Tiên): located south of Phong Nha, this grotto is 520 m long.
- Khe Ry Grotto (Hang Khe Ry): located south of Phong Nha. The grotto is where the long Khe Ry stream (13.817 km or 8.585 miles) runs out
- Khe Thi Grotto (Hang Khe Thi): a stream originating from the mountain, running underground the grotto