Mexico's giant crystal cave got it's name from the stunning 10 meter long crystals that were found inside. The cave is extremely hot with air temperatures reaching up to 58 °C (136 °F) with 90 to 99 percent humidity. Due to these factors, the cave is relatively unexplored. Without proper protection people can only endure approximately ten minutes of exposure at a time. The cave lies on an ancient fault and there is an underground magma chamber below the cave. The magma heated the ground water and it 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.
Bonus Picture: A science team explores the cave.
BBC Docu - How Earth Made Us