Last chance to see: Video of Mexico's Naica Cave of Crystals (Previously, and previously.) [more inside]
Crystal Palace. "Cavers in Mexico confront extreme conditions and find extraordinary beauty." (Previously.)
The gypsum crystals in the Cave of Crystals at the Naica mine in Chihuahua, Mexico, are some of the largest and most spectacular in the world. [Last link is a .wmv]
50 foot long single spar crystals found in a Mexican cave 1,000 feet below the surface! Smithsonian has links to other related sites. This one has pictues. More pictures can be found in the April 2002 print issue of Smithsonian.