The Naica Mine of the Mexican state of Chihuahua is a working mine that is best known for its extraordinary selenite crystals. Located in Naica in the municipality of Saucillo, the Naica Mine is a lead, zinc and silver mine operated by Industrias Peñoles, Mexico's largest lead producer. Caverns discovered during mining operations contain crystals of selenite (gypsum) as large as 4 feet (1.2 m) in diameter and 50 feet (15 m) long.
Cave of the Crystals 
The Cave of Crystals (Cueva de los Cristales) is a cave approximately 1,000 feet (300 m) below the surface in the limestone host rock of the mine. The chamber contains giant selenite crystals, some of the largest natural crystals ever found. The selenite crystals were formed by hydrothermal fluids emanating from the magma chambers below. The cavern was discovered while the miners were drilling through the Naica fault, which they were worried would flood the mine.
The caves are closed to the public.
The Cave of Swords 
The Cave of Swords is another chamber in the Naica Mine, also containing gypsum crystals but each only about a meter long.
- García-Ruiz, Juan Manuel, 2007, Formation of natural gypsum megacrystals in Naica, Mexico, Geology, 35(4): p. 327 doi:10.1130/G23393A.1
- "Naica: largest lead producing mine in Mexico"
- National Geographic, 2008. Cavern of Crystal Giants
- National Geographic, 2007. Giant Crystal Cave's Mystery Solved
- Naica mine at Peñoles
- Naica Mine at Mindat.org
- Naica Project, comprehensive website on the Crystal Caves with geology, history, photos and videos
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