If you come to northern Minnesota, a visit to at least one iron mine should be manditory. As luck would have it, there are two iron mines in Minnesota's State Park inventory. The Soudan mine is the underground one.
At one time, this area of Minnesota produced more iron than anywhere else in the world; this mine tells that story. You ride a half-mile underground in the same mine cage used by the miners 45 years ago (a word of warning... the faint of heart may be challenged by the elivator ride). Upon reacking the 28th and lowest level of the mine, the guide takes you 3/4 of a mile horizontally, on a mine train, where you reach the actual work site. You'd think it would be claustrophobic down there, but it really quite open and airy. Did I mention it was chilly? 50 degrees F when you aren't moving; 45 degrees F on the train. When we visited, it was 90 degrees on the surface... it sure felt good down below!