In Mexico's Naica mine, 900 feet below the surface, are gypsum crystals that are 36 feet long, and up to 55 tons.
The mine is right next to a magma chamber, so it's 120 degrees Fahrenheit and 90 percent humidity in there. Visitors have to wear cooling suits with frozen gel packs sewn in, and carry backpacks with chilled air to breathe.
Very cool stuff! Check out video on the original article.