Salina Turda: salt mines turned subterranean history museum. "What was once an enormous salt mine in Turda, Romania, has now been carefully renovated by the regional Cluj county council into the world’s first salt mining history museum. The Salina Turda salt mines were excavated in the 17th century, proving a crucial source for salt that brought the Romans much wealth. Today, the Durgau lakes at the mine’s surface – responsible for much of the salt deposits in the area – are popular tourist attractions that guarantee a steady flow of visitors all year around. A trip down the vertical shafts that once transported thousands of tons of salt will slowly reveal the immense scale of the excavated earth, made blatantly clear upon reaching the very bottom of the mine which is covered in a sand-like layer of salt."
Inside a mile-deep open-pit copper mine after a landslide. Flickr set and other links in the article.
Hallstatt, Austria, besides being idylic, is a UNESCO World Heritage Site, and is historically fascinating: A Bronze Age cultural center, with a 2,500-year-old salt mine (the world's first); beautiful ice caves; and a Catholic cemetery so small that the dead were regularly disinterred after a time, their skulls painstakingly identified and decorated and stacked in an ossuary.
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.