When we were in Krakow, just before Christmas, we went to visit the famous salt mine at Wieliczka. The mine was in operation from the C13th till 1996, and is now a UNESCO World Heritage Site. 287 km of passages and chambers have been cut through the solid, greyish salt, which you can see on the floor of this corridor. Everything that isn't made of salt seems to be made of wood. You can only visit as part of a guided tour, but it's a great day out.
