“Past the hand of dog, watch out for dinosaurs,” says a voice in the dark. I recognize ... the park’s namesake show cave, Hang Phong Nha, which workers light up like a psychedelic rock ...

The world’s largest cave is over five miles long - so big that even a Boeing 747 could fit inside it. Hang Son Doong is in Phong Nha-Ke Bang National Park in Vietnam, near the border with Laos ...