Wieliczka and Bochnia Royal Salt Mines by Els Slots
One of the most incredible places I have ever seen--truly breathtakingly unbelievable to walk into the cathedral 300 feet below ground level. The good news is that after walking down you get to ride up although be it in a cramped elevator--not good if you have claustrophobia. Had a private tour guide in English who was very knowledgable and wise. Highly recommend visiting this site.