Antelope Canyon, Arizona, United States
This canyon was formed by millions of years of flowing water that carved out a deep, yet narrow crevice. Since significantly less light makes it down to the deeper depths, the walls often appear to be different colors.
Sadly, the mining company that owns the caves could no longer afford to keep the massive pumps going to clear the chamber. I believe the pumps were shut down a few years ago and the cavern is refilled with water. The crystals are growing once again, but no one will ever see it.