The BBC's film crew makes a documentary about the world natural heritage site Phong Nha-Ke Bang. (Photo: sggp.org.vn)

The crew filmed Phong Nha-Ke Bang, the world's largest caves, the cave ecosystem inside, the limestone mountain and tropical forest ecosystems. Thomas Webb, the BBC's producer, said they were amazed by the magical natural beauty of the caves, primeval forests, and diverse and unique ecosystems there.

The national park of Phong Nha-Ke Bang has been twice recognized by UNESCO as a world natural heritage site.

The documentary is expected to be released by the BBC in the fourth quarter of 2022.