Crystal Cave in Mexico
Its main chamber contains the largest selenite crystals ever found, some of them reaching 11 meters in length, 4 meters in diameter and about 55 tons in weight. The conditions here are extreme, with a constant temperature of 58 °C and high humidity, proving to be a dangerous environment for those who are exposed more than 10 minutes without proper equipment.
The crystals are said to be 500,000 years old and were formed from the underground magma which maintained the water at a stable temperature of 50°C, saturating it with minerals, including gypsum. This process allowed the selenite crystals to grow to the unbelievable sizes that we see today. (Source)