Very cool place, we were there a number of years ago. The other part that Brian didn't mention is that attached to the mine is a high energy physics lab that was installed there to take advantage of the existing cavern 1/2 mile underground in old, stable rock. Among the experiments they do there are studies on subatomic particles that are fired at the target in the mine from the accelerator at Fermilab in Chicago, traveling some 450 miles through the solid rock in between. When we were there they were just finishing work on the lab but it's been in operation for a while now. Cutting edge physics in the woods of northern Minnesota - who would have thunk it?
Soudan Underground Mine High Energy Physics LabUniversity of Minnesota Lab page