Minnesota has only one area of carbonate rocks in the south eastern corner of the state. A small area of Devonian limestone surrounded by a ring of Ordovician limestones and dolostones. Here is the karst area with both show caves of the state.
The rest of the state is made of siltstone, sandstone, intrusive and estrusive rocks. Some of the rocks are 3.6 billion years old. The northern half of the state belongs to the Canadian shield. This rocks are not soluble, so they have no caves at all.
But the norteastern corner of Minnesota has another type of underground sights. This area is called the Iron Range, a hillcountry which has numerous iron ore deposits. But there are also various deposits of nickel, lead, gold, silver, and zinc. In the mid 19th century the area had a small gold rush, but the real treasure of the area is iron ore. The three ranges Vermilion Range, Mesabi Range and Cuyuna Range were the economic base of Minnesota and the main source of iron ore for the nation's steel mills for more than 50 years.
The iron was first mined in open casts. A fine example is Hill Annex Mine, which is the biggest opencast of the world. The oldest and deepest iron mine of Minnesota is located in Vermilion Range and is called Soudan Mine.