Cave of Crystals - Naica, Mexico
980 ft below the surface lie some of the largest natural crystals ever found, these giant selenite crystals were formed above a magma chamber, the magma below heated the ground water which became saturated with minerals, including large quantities of gypsum. The hollow space of the cave was filled with this mineral-rich hot water and remained filled for about 500,000 years; during this time, the temperature of the water remained very stable at over 50 °C. This allowed crystals to form and grow to immense sizes.
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.
Below is a video from a BBC TV show about these amazing formations.