Phong Nha - Ke Bang can be compared to a huge geological museum thanks to its complicated geological structure with different categories of stone including sandstone, quartz, schist, siliceous limestone, granite, granodiorite, diorite, applet, pegmatite, etc. Phong Nha-Ke Bang is the oldest and largest tropical karst formed 400 million years during the main geological periods of the Earth, bearing the original topographic and geologic characteristics. Experiencing major tectonic phases, high mountain ranges and the depressed sedimentary basins were formed. These fluctuations have also contributed to the diversity of geology, topography, geomorphology.

Phong Nha - Ke Bang karst mountains can provide a lot of valuable information about the Earth's prolonged geological process through various periods, from the Ordovician Age - Silurian Age (about 463.9 - 430 million year ago) to the Quaternary Age (1.75 million years ago). It is also noteworthy to mention a geological characteristic � the system of underground rivers and grottos in limestone mountains.

In the non-karst geomorphologic area, there are many low mountains covered by a floristic carpet. The erosion has created a number of abrasion-accumulation terraces along the valleys of the Son and Chay rivers and at the margins of the central limestone massifs. The transition terrain consists of a diversity of rock intercalated by limestone mountains.

Besides the historical value of geology, topography, geomorphology, Phong Nha - Ke Bang is also favoured with the mysterious and majestic landscapes by nature. The Phong Nha - Ke Bang Natural Park still hides various myths of nature. It also has spectacular karst caves formed for hundreds of millions of years.

The system of Vom caves is over 35km long, rising from Ruc Ca Roong Cave located at a height of 360m above sea level and ending with Vom Cave. The system runs south and north. Ruc Ca Roong River sometimes hides in mountains, sometimes appears in narrow and deep valleys, and flows into the Chay River at the entrance of Vom Cave.

The system of Ruc Mon caves that lies in the district of Minh Hoa is also a large cave. However, the information about this system is not abundant because few surveys have been conducted in this area so far.

Some typical caves and grottoes in Phong Nha - Ke Bang are Phong Nha, Tien Son and Thien Duong grottoes, Toi, En and Khe Ry caves. Especially, Son Doong Cave is considered as the largest cave in the world with 200m in height, 200m in width, at least 8.5km in length.

With its characteristics of topography, climate, soil and hydrography, Phong Nha - Ke Bang boasts a rich, diversified and unique flora. Investigations show that Phong Nha - Ke Bang has a large tropical forest floor. It covers 93.57 percent of the natural area, of which nearly 83.74 percent is covered by primeval forests. The diversity of rare and precious species of animals and plants in Phong Nha - Ke Bang is corollary of its natural conditions, and is a feature of the forest ecosystem.

Phong Nha � Ke Bang has been declared a world natural heritage by UNESCO twice in July 2003 in the 27th meeting in Paris (France) and July 2015 in the 39th meeting in Bonn (Federal Republic of Germany).