The Soudan Underground Mine is known as Minnesota's oldest, deepest, and richest iron mine, and now hosts the Soudan Underground Laboratory. In the late 19th century, prospectors searching for gold in northern Minnesota discovered extremely rich veins of hematite at this site, often containing more than 65% iron. An open pit mine began operation in 1882, and moved to underground mining by 1900 for reasons of safety. From 1901 until the end of active mining in 1962, the Soudan Mine was owned by the United States Steel Corporation's Oliver Iron Mining division. By 1912 the mine was at a depth of 1,250 feet (381 m). When it closed, level 27 was being developed at 2,341 feet (713.5 m) below the surface. US Steel then donated the Soudan Mine to the State of Minnesota to use for educational purposes.