Cave of the Crystals, Mexico

The Crystal Cave in the Naica mine.
 This cave contains gigantic, sword-like gypsum crystals. It is 300m underground in the Naica Mine in the Mexican state of Chihuahua.

 It was discovered by two brothers drilling for lead and silver.The enormous crystals are believed to have formed when gypsum-saturated groundwater flowed through the caves, and was heated and cooled by hot magma below. Some of the largest crystals may be over 500,000 years old.