Fortress of Solitude is in…Mexico???

yep! it can be found right here. Here’s a quote!

Mexico’s Cave of Crystals contains some of the world’s largest known natural crystals—translucent beams of gypsum as long as 36 feet (11 meters). A new study says the gems reached their vast sizes thanks to a peculiar combination of consistent volcanic heat and a rich watery mixture.

Source. Thanks, Kamen!