Michigan's Keweenaw Peninsula is the most important locality in the world for pure native copper. Mining began in 1845 and continued until 1968. Over 5 million tons of refined copper were recovered from Michigan's copper mines. In the early 1900's, it led the world in copper production and "Lake Copper" was the purity standard to which other copper was compared. Long before the mining operations of the past century began in the Keweenaw, pre-historic peoples mined copper from the mineral-rich veins.