"An enormous shaft of sunlight plunges into the cave like a waterfall. The hole in the ceiling through which the light cascades is unbelievably large, at least 300' across. The light, penetrating deep into the cave, reveals for the first time the mind-blowing proportions of Hang Son Doong. The passage is perhaps 300' wide, the ceiling nearly 800' tall: room enough for an entire New York City block of 40-story buildings. There are actually wispy clouds up near the ceiling."
Hang Sơn Đoòng ["Mountain River cave"] was found in 1991 by local Vietnamese man Ho Khanh. Located in Phong Nha-Kẻ Bàng National Park on Vietnam's border with Laos - a 6-hour hike from the nearest road - the cavern (photos here, videos here) is the largest in the world (pictured above, note size scale of explorer in 3rd image). It was surveyed in 2009 (interactive graphic here) and contains not only a forest of 30m-tall trees, but an underground river, a 61m wall of calcite, cave pearls, stalagmites more than 70m tall, gigantic skylights in the limestone ceiling, and a chamber 5km long, 200m high, and 150m wide.
The cave remained so well-hidden that even Khanh forgot where he had found it 20 years earlier as a young boy. Adam Spillane of the British Cave Research Association explains, "The terrain in that area of Vietnam is very difficult. The cave is very far out of the way. It's totally covered in jungle, and you can't see anything on Google Earth. You've got to be very close to the cave to find it. Certainly, on previous expeditions, people have passed within a few hundred meters of the entrance without finding it."
Official website: http://www.sondoongcave.org/
Coordinates: 17°27'25.88"N 106°17'15.36"E