A cave in the Annamite Mountains of Viet Nam contains a river and jungle and is large enough, in spots, to hold a skyscraper.
Hang Son Doong Cave is part of a network of about 150 caves in central Vietnam near the Laotian border. The husband and wife team of Howard and Deb Limbert first discovered it in 2009 but only recently returned to scale a huge calcite wall and try to find the cavern's end.
See the details at National Geographic along with an interactive graphic of the river cave and its path.
Discovery: Hang Son Doong Cave in Viet Nam Big Enough To Hold A Skyscraper