The biggest cave in the world

I was in the biggest(accessible to the public) cave in the world  today, 250 metres in diameter and almost 200 metres high.  It covers a floor space of 5 hectares and you could fit nine Notre Dames into its space (sorry it's a French cave, so the comparisons are French).  I didn't even try taking any photos. A tablet phone is just not up to the magnificence of this place.   But to give an impression of just how huge it is  here's a photo of a full size hot air balloon being flown within it!  (There is also a video here but the first ten minutes of it are about the logistics of getting the stuff up the mountain (before there was a road to the entrance) and into the cave.

The system was discovered in 1950 and the large cavern in 1953.  The whole system has still not been completely explored and has its own ecosystem, half a dozen blind insect species, several of whom are endemic to this cave system. Big wow factor!