When Hồ Khanh, a 16-year-old in the Quảng Bình Province in Vietnam, was taking a walk in the lush forests of the Phong Nha-Kẻ Bàng National Park in 1991, he wasn't looking for anything in particular. He may have been seeking a little peace and quiet, perhaps, or some time for solitary reflection and relaxation in the heart of the forest. What he found was mind-blowing.
As he walked, he took a misstep and, like something out of a fantastical movie, the jungle floor opened up under Khanh's feet. He jumped back as the earth fell away from his feet.
When he peered down into the chasm where he'd been standing just a moment before, he could see only a deep darkness.
Khanh had just unwittingly discovered a secret entrance to a cave that had been hidden from mankind for literally millions of years.
Not only that, but the cave turned out to be the largest cave on the whole planet.
It was five times larger than the biggest known cave, Hölloch in Muotathal, Switzerland.
The cave was named Sơn Đoòng Cave. Since most large caves contain some evidence of a shrine, living quarters, or ancient art, it's entirely possible that Khanh was the first human on the earth to lay eyes on this cave.
The cave contains some of the earth's tallest stalagmites, up to 230 feet tall.
Amazingly, researchers didn't fully explore the cave until 2009 when a group of scientists from the British Cave Research Association went in.
The cave contains a fast-flowing river which served as Sơn Đoòng's namesake - it means "mountain river" in Vietnamese.
The water's moving concentration of calcium salts created "cave pearls," round speleothem the size of baseballs, some of the largest in the world.
On the other side of the cave pearls, Sơn Đoòng also contains a 200-foot high calcite wall which the spelunking researchers nicknamed "The Great Wall of Vietnam."
The cave's first guided tours began in 2013. The guided tours cost $3,000 each and require a day and a half trek to reach the cave, a descent 500 feet down and a walk through all 5.5 miles of the Sơn Đoòng cave.
Credit: Messy Nessy Chic