La Cueva de los Cristales, or The Cave of the Crystals rests nearly 1000 feet below one of Mexico's most productive mines in Naica mountain in the Chihuahuan Desert. In 2000, the cave was discovered by two miner brothers who were excavating a new tunnel. The cave contains some of the largest naturally occurring crystals ever recorded – some even measure up to 36 feet long and weigh as much as 55 tons!
The cave is extremely hot and humid (around 120 degrees Fahrenheit and 90 to 100 percent humidity), making the cave nearly impossible for humans to survive in for longer than a mere two hours. But these conditions made the caves so ideal for the crystals to grow to such large proportions. Once the caves were discovered and drained (allowing people in), the crystals stopped growing. It's believed that if draining was stopped, the crystals would begin to grow again, and apparently there is no limit to the size they could become.
Multiple other crystal caves exist below the Naica mine, specifically Sword Cave, Candle Cave, and Queen's Eye Cave. Back in 2006, the Naica Project was formed in an effort to further explore and promote the natural wonders.