The discovery of a boulder of high-grade iron ore by Leonidas Merritt in 1887 during a survey for the railroad, which he brought to Duluth, the Merritt brothers (“the Seven Iron Men”) established the Mountain Iron Mine in 1892 Mountain Iron, Minnesota which represents the beginning of the exploitation of the Mesabi Range iron ore in the Iron Range of northeast Minnesota. By 1893 the Merritts had claims on a significant portion of the Mesabi Iron Range and had built the Duluth, Missabe & Northern Railway DM&IR. Financial conditions forced them sell their shares to John D. Rockefeller who later sold  to Andrew Carnegie. The early development was as an underground mine, but open cast mining soon proved to be a better choice because of the soft, shallow ore deposits. The Mesabi Range and nearby Vermilion Range led Minnesota to become the nation's largest producer of iron ore and the United States to lead the world in steel production. This capacity is considered to have been a major factor in America's ability to contribute to World War II. It also played a major role in the financial success of Andrew Carnegie and U.S. Steel. Carnegie returned some of his fortune to the communities by funding 2500 public Carnegie Libraries across the country, including 64 in rural Minnesota.
The extraction of ore in the region also contributed to the tiny port city of Duluth thriving and becoming the leading port in the United States (by tonnage) in the early 20th century.