http://www.dailymail.co.uk/sciencetech/article-2441450/Er-Wang-Dong-cave-China-huge-weather-system.html

Adventurers have stumbled across a cave so enormous that it has its own weather system, complete with wispy clouds and lingering fog inside vast caverns.

A team of expert cavers and photographers have been exploring the vast cave system in the
Chongquing province of China and have taken the first-ever photographs of the natural wonder.

They were amazed to discover the entrance to the hidden Er Wang Dong cave system and were stunned when they managed to climb inside to see a space so large that it can contain a cloud.

That's pretty cool, actually.