The Mesabi Iron Range is an elongate trend containing large deposits of iron ore, and the largest of four major iron ranges in the region collectively known as the Iron Range of Minnesota. Discovered in 1866, it is the chief iron ore mining district in the United States. The district is located in northeast Minnesota, largely in Itasca and Saint Louis counties. It was extensively worked in the earlier part of the 20th century. Extraction operations declined throughout the mid-1970s but rebounded in 2005. China's growing demand for iron, along with the falling value of the U.S. dollar versus other world currencies, have made taconite production profitable again, and some mines that had closed have been reopened, while current mines have been expanded.