Wieliczka Salt Mine
The Wieliczka Salt Mine, about 30 minutes from Krakow, is definitely worth a day tour. The mine opened in the 13th century and had been mined until 1996. Almost 600 years of mining. It reaches a depth of 327 meters and has 178 miles of tunnels (stay close to your guide). Inside the mine there are the amazing sculptures, bas-relief panels and chandeliers all craved or made from rock salt. There is an underground lake and there used to be boat tours through the tunnels, but they no longer run.
One of the things I was most impressed with was the unbelievable amount of wooden timbers transported into the mines to create these vast rooms, grand staircases and underground tunnels. It looks like a whole forest of trees was used down there.