It was 1991. A Vietnamese logger named Ho Khanh was hiking through the jungle near his home in Phong Nha, in central Vietnam. Drawn by the sound of rushing water, he came to an opening in a cliff. A strong wind blew clouds out of the cave—an intimidating site, and enough to keep Ho Khanh from searching deeper—so he turned around and went about his business. That was the unceremonious discovery of Sơn Đoòng, the largest cave in the world. "It's five miles long, 650 feet high, and 500 feet wide," writes Ken Jennings for Traveler. "It could hold an entire city block of Manhattan, including 40-story skyscrapers."
And somehow, it managed to remain a secret for centuries. But by 2005, the surrounding area had become something of a destination for spelunkers, and Ho Khanh realized he might have been on to something. It took three years of searching, of tracing his steps, before he was able to find the opening again, but when he did in 2009 with a British caving expedition, they became the first people to tackle Sơn Đoòng. Only a thousand or so people have done it since—including us. This is our five-day trek into the center of the earth.