Mexico's Cave of Crystals stunned geologists when it was first discovered in 2000. The underground chamber contains some of the largest natural crystals ever found - some of the selenite structures have grown to more than 10m long. Professor Iain Stewart got a rare glimpse of the subterranean spectacle while filming for the new BBC series How the Earth Made Us.
We kept on being told how difficult it was going to be to film in the Naica Cave, but nothing really prepares you for the extremes of that cavern.
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....