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These mind-boggling crystals, the largest 36 feet long, were found in 2000 near the town of Delicias, Chihuaha state, Mexico. The caverns are threatened with excavation by miners, but geologists are hoping to preserve them.

From <a href="http://news.nationalgeographic.com/news/2007/04/070409-crystal-cave.html" target="_blank">National Geographic</a>