India, July 10 -- Ho Khanh was on a mission to hunt for a rare jungle tree Agarwood near the border of Vietnam and Laos in 1990 when a heavy storm pounded down on him, driving the Phong Nha native to find shelter under a cliff. Despite keeping dry, the mountainous columns of fog and cold air that flowed out of what looked like a cave terrified him, making a serious impression he couldn't shake.

It wasn't until 17 years later, when Khanh was on a trip in the Hang En area with Howard Limbert of the British Vietnam Caving Expedition team, that he mentioned the decades-old experience he had - capturing the intrigue of his team. While it took some time to retrace his steps, Khanh managed to lead the team to the exact spot in 2009. They quick...