Hang Son Doong is the world's largest cave. Located in central Vietnam it wasn't discovered until 1991 and then finally explored in 2009. 3 kilometers from the entrance the roof has collapsed and the rainforest has invaded- untouched until now. There are commercial tours taking people into the cave system at this point.
It is, of course, a worry as to how the commercialization will be monitored to insure the fragile environment is not damaged. Have you seen any reports on the damage that commercial enterprise has inflicted on Mt. Everest?
Additional information on Son Doong here as well as on Wikipedia. There are also numerous videos on YouTube.
iPad users click here to view the video