Discovered in Vietnam in 1991, wasn't explorer until just a few years ago. It's 5x larger than the next largest cave (also in Vietnam). Probably over 2 million years old, and appears to have never been inhabited. It features stalagmites up to 70m tall, a river and waterfall: and if you're feeling adventurous you can go on a tour there.
See more at : http://www.sondoongcave.org/