Guinness World Records lists Hang Son Doong as the world's largest known single cave passage, and sunlight pouring through its collapsed roof feeds a forest that would not exist in an ordinary cave. in 1990, a Vietnamese farmer hiding from a storm found a cave where sunlight grew a 165 foot forest above an underground river. The farmer, Ho Khanh, had wandered into the mountains of central Vietnam looking for shelter when he heard rushing water below the ground and saw mist rising from somewhere he could not see.
He did not explore further that day, and for years afterward he could not find his way back to the spot. It took until April 2009 for him to guide a team of British cavers to the entrance, and only then did survey work begin that revealed the true scale of what Khanh had found, a cave now recognised by Guinness World Records as the world's largest known single cave passage.
