The Cave of the Crystals is a little-explored cave in Naica, Chihuahua, Mexico. Lying 300 metres (1,000 ft) below the surface and connected to the Naica Mine, the main chamber contains some of the largest crystals ever found. The largest of these gypsum formations is 12 m (40 ft) in length, 4 m (13 ft) in diameter and 55 tons in weight.Photograph: Alexander Van Driessche