WAIST-DEEP in cool river water on a sweltering June afternoon, we waded towards the entrance of the immense Hang En cave in the Quang Binh province of central Vietnam. There, we donned our hard hats and headlamps and silently entered single file, darkness enveloping us, just the light of our flashlights illuminating our path. A few hundred feet in, we reached a mountain of boulders. As we scrambled up, the light became more intense as we gained height. On reaching the summit we were stopped dead in our tracks by the view before us - the cave's gigantic main cavern.

At 300 feet in height and 600 feet across, the cavern is big enough to fit a Boeing 747 with room to spare. The space was flooded with rays of natural light coming in from an arch high above us. The beams of light illuminated a yellow sand beach hundreds of feet below, surrounding a calm turquoise pool.

Two nights before I had been having dinner with Howard Limbert, a gregarious 57-year-old cave specialist who left his job in England as a biomedical scientist in 2012 to devote his life to exploring the caves of Vietnam.