This is the underside of a cenote, a large sink hole that has a pool of water on the bottom.
There are tree roots growing down 100' and the light is bouncing up off the water below onto the "roof".
The humidity was 100% and it was around 100 degrees F allowing the light to illuminate the mist...
I was drenched in sweat buy the time I climbed back out and didn't dare change lenses in there!