World's largest cave: Hang Son Doong

"See the speck inside the glowing area in the picture above? It's a person, standing in a cave called Son Doong.
Son Doong is in Vietnam, near the Laos border, in Phong Nha-Ke Bang National Park. And it is enormous -- easily, the world's largest known cave -- extending over five and a half miles long. (The next largest cave so far discovered, Deer Cave in Malaysia, is 'only' about a mile long.)
Son Doong was discovered in 1991 by a local man named Ho-Khanh but was left unexplored for nearly two decades afterward, in large part because the cave emitted a strange whistling sound from within. In 2009, a formal expedition team entered the cave only to find themselves stopped by a 200 foot wall, believed to be insurmountable. But two years later, a team comprised of many of the same explorers returned and successfully scaled the wall.
What they discovered was that the cave was much larger than they previously believed. At one point, Son Doong contains a cavern roughly 650 feet high and 500 feet wide. (For context's sake, the Leaning Tower of Pisa is, approximately, a mere 185 feet tall and 50 feet wide at its base. One could stack thirty Towers three high and 10 across, and still have open space at the top.) Even passageways are mindbogglingly large. One cave visitor, Mark Jenkins, writing for National Geographic, describes it: 

'[There are] building-size blocks of stone that have fallen from the ceiling and crashed onto the cave floor. I crane my head back, but the immensity of the cave douses my headlamp’s tiny light, as if I were staring up into a starless night sky. I’ve been told I’m inside a space large enough to park a 747, but I have no way to know; the darkness is like a sleeping bag pulled over my head.'

The cave has a number of other features. The whistling noise is caused by an underground river which gives the cave its name; in Vietnamese, the cave is named 'Hang Son Doong,' or 'mountain river cave.' According (again) to National Geographic, the cave is rife with poisonous centipedes, while monkeys have found a way to enter the cave from its top, nearly 1,000 feet above the cave's floor. And certainly there are dozens if not thousands of other mysteries yet to be discovered within the cave's incredible expanse."

- Found through Now I Know, Images from National Geographics website