The Wieliczka Salt Mine near Kraków is 2 miles in length and 450 feet deep. It is not active presently, and instead, it is used as a discovery experience to learn about the history of mining.
Walking down to the bottom level you pass 20 chambers with intricately designed mythical statutes chiseled out of rock salt on your nearly three hour tour.
You visit the largest underground cathedral in the world which continues to hold services. There is a special entrance to get there.
i have toured the working American Salt Mine in Groveland, NY, and this was an entirely different experience.