The Cave of Crystals, located in the working Naica Mine of Chihuahua, Mexico, has captivated the world since it was first discovered in 2000 by two brothers drilling in the Naica lead and silver mine. Until you notice the orange-suited men clambering around, it's hard to grasp the extraordinary scale of this underground crystal forest which are some of the largest natural selenite crystals in the world, with stunning crystal formations measuring more than 30ft (9.2m) in length and over 4ft (1.2m) in diameter. They are formed from groundwater saturated in calcium sulphate which, warmed by an intrusion of magma about a mile below, began filtering through the cave system millions of years ago.