While excavating a tunnel under Mexico's Naica Mountain, silver miners were stunned to discover a cavern full of luminous, elongated underground crystals. Composed of the mineral gypsum, some of the crystals extended as far as thirty-six feet into the air. What accounted for these amazing mineral structures? Millions of years ago, volcanic eruptions released a large amount of mineral-rich water into the cave, now known as the Cave of Crystals. One of those minerals was anhydrite, which slowly changes into gypsum when exposed to water. Originally, the cave was too hot for this change to take place. Eventually, however, the temperature cooled to about 136°F, and gypsum crystals began to form. This process continued as the temperature and other conditions in the cave remained constant for thousands of years.
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