A doctor examines a child after he disembarked from the German Navy ship Schleswig Holstein at the Reggio Calabria harbor, Italy, Tuesday, June 16, 2015. European Union nations failed to bridge differences Tuesday over an emergency plan to share the burden of the thousands of refugees crossing the Mediterranean, while on the French-Italian border, police in riot gear forcibly removed dozens of migrants. (AP Photo/Adriana Sapone)

Cave of the Crystals or Giant Crystal Cave is a cave connected to the Naica Mine 300 metres (980 ft) below the surface in Naica, Chihuahua, Mexico. The main chamber contains giant selenite crystals (gypsum, CaSO4·2 H2O), some of the largest natural crystals ever found. The cave's largest crystal found to date is 12 m (39 ft) in length, 4 m (13 ft) in diameter and 55 tons in weight. The cave is extremely hot with air temperatures reaching up to 58 °C (136 °F) with 90 to 99 percent humidity. The cave is relatively unexplored due to these factors. Without proper protection people can only endure approximately ten minutes of exposure at a time.