The Homestake Mine has been a solid part of the Black Hills economy for quite a while. In addition to being a gold mine, today it is also used as an underground laboratory in it's deeper recesses. Some tunnels are 8,000 feet deep.
My wife's favorite part was the sign explaining the cyanide process for gold recovery. If you go to the visitor center, be sure to look at the various mining equipment around, ranging from power drills to mine trains, elevator flywheels, etc. At a park just up the hill is an old stamp mill. Several museums are in the area as well. You could definitely spend some time here!