The Cave of the Crystals holds some of the largest natural crystals ever found. The biggest measured is 11.4 metres long, about 5 cubic metres in volume, with an estimated mass of 12 tonnes. They are selenite, a form of gypsum, and they grew for at least 500,000 years.
The chamber sits 300 metres down, connected to the Naica mine in Chihuahua, and it is about 109 metres long. It was found in April 2000 by two brothers, Juan and Pedro Sánchez, drilling in the mine.
How it formed is a piece of chemistry that had to go right for an extraordinarily long time. Naica lies on a fault above a magma chamber three to five kilometres below the cave. The magma heated groundwater saturated with sulfide ions. Cool, oxygen-rich surface water met that heated water and, crucially, did not mix with it, because the two differed in density. Oxygen diffused slowly across the boundary, oxidising sulfides into sulfates, which precipitated as anhydrite. When the cave finally cooled below 56 degrees Celsius, that anhydrite dissolved and gypsum crystallised in its place, extremely slowly, for half a million years.
A second cave found in 1910 above this one, the Cave of Swords at 120 metres depth, holds spectacular crystals only about a metre long. The likeliest explanation is that temperatures there fell far more quickly.
You cannot visit. Unflooded, the cave reaches 58 degrees Celsius at 90 to 99 percent humidity, which prevents cooling by sweating and limits unprotected exposure to roughly ten minutes. As of October 2015 the mine had reflooded and the chamber filled again with the water the crystals need to keep growing.
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