Wieliczka Salt Mine in Poland has been in operation for 800-odd years. Needless to say, this has given the miners plenty of time to carve some pretty amazing things. More photos here and here. Videos here and here. Virtual tour here. Wikipedia.
posted on Aug 16, 2007 - View this thread

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]
posted on Aug 3, 2007 - View this thread