Cave of the Crystals

Cave of the Crystals or Giant Crystal Cave (Spanish: Cueva de los Cristales) 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. The cave was discovered by the brothers Eloy and Javier Delgado. Source: Wikipedia 

Water-clear selenite crystal "floater" from the Naica Mine. Size: 18×14×13 cm., weight 2.6 kg.