Look at the size of that passage! Cavers around the world are now salivating and wondering how to get there. Photo gallery and story at the link.
It's Hang Son Doong or Mountain River Cave in northern Vietnam near the Laos border. This, and numerous other caves, are protected in Phong Nha-Ke Bang National Park in the Annamite Mountains. These caves formed in a limestone massif from the flow of underground rivers - their size is what makes them so impressive.