The underground chamber of Naica Crystal cave (Mexico) contains some of the largest natural crystals ever found - some of the selenite structures have grown to more than 10m long.
Miners working in the Naica silver mine broke through the walls of the cavern and were astounded to discover these enormous crystals - the biggest anywhere on Earth.
But when the first people went in to explore, they were almost overcome by the conditions -It's about 50C in there, but it's the virtually 100% humidity added on top that makes it a potential killer.
That combination means that when you breathe air into your body, the surface of your lungs is actually the coolest surface the air encounters. That means the fluid starts to condense inside your lungs - and that's really not good news.
(Thanks to Charlie Inman for the heads up, text from BBC)