The Cave of the Crystals in Chihuahua, Mexico is not only a stunning geological wonder but also incredibly dangerous to visit.

"It seems like walking into an oven. You're hit by the heat and humidity, it's simply unimaginable, it sticks to your skin. You go in fighting temptation to go back out again, while your body sweats, ...

These huge gypsum crystals in Mexico’s Naica caves, some up to 12m long, have been growing for tens of thousands of years -- but scientists have only now discovered what’s inside them ...