I've visited a lot of underground caves and mines over the years, but this was without a doubt the most spectacular. Wieliczka Salt Mine is famous for its underground church, with every sculpture and chandelier carved out of salt. A UNESCO World Heritage Site, if you have the time it's well worth a half day trip from Krakow.
Claire and I had already decided that we didn't want to visit Auschwitz, neither of us felt emotionally ready for such an excursion, and Claire had family who died there. We did want to do a trip outside of Krakow though, so after speaking to my Uncle we decided on Wieliczka Salt Mine. He raved about it and after looking at photos online we were sold.