Son Doong Cave—Wind River Cave—is located in the impenetrable mountainous jungles along the old Ho Chi Minh trail on the Vietnamese side of the Vietnam-Laos border. To get to the cave, you have to slog down a jungle mountainside, up a river bed, then through another vast cave. The exit on the far side of this first cave opens on the otherwise inaccessible valley that is the starting point for entering the Son Doong Cave. We were told by our expedition leader that fewer people have been to Son Doong than have been to space.

Within the cave, there are vast areas open to the jungle above. These never scaled heights let in shafts of light, unusual flora, and the occasional monkey descending on vines.

To get the picture, think one part Avatar, one part the glittering caves of Aglarond from the Lord of the Rings, one part Edgar Rice Burroughs’s Tarzan of the Apes (the original books, not any of the film versions)—with an added pinch of Eustace’s adventures on the Dragon Island in the third book of the Narnia series.