From UNESCO:

The karst formation of Phong Nha-Ke Bang National Park has evolved since the Palaeozoic (some 400 million years ago) and so is the oldest major karst area in Asia. Subject to massive tectonic changes, the park’s karst landscape is extremely complex with many geomorphic features of considerable significance. The vast area, extending to the border of the Lao People’s Democratic Republic, contains spectacular formations including 65 km of caves and underground rivers.

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