Cueva de los Cristales
Wiki says that the Cueva de los Cristales is a cave connected to the Naica Mine 300 metres (980 ft) deep in Chihuahua, Mexico.
The main chamber contains giant selenite crystals, some of the largest natural crystals ever found.
The cave is relatively unexplored due to the extreme temperatures and high humidity. Without proper protection people can only endure approximately ten minutes of exposure at a time.
Image Credit: National Geographic
... I've seen vugs of bright, shiny jewels and stood in caves completely surrounded by crystals but, I've never seen anything like this.
And those extreme temperatures, I know exactly what they mean.
Here's a link to an excellent photo set which will give you a better idea of the scale.