Baltimore County is a county located in the northern part of the U.S. state of Maryland. In 2004, its population was estimated to be 763,181. It is part of the Baltimore-Washington Metropolitan Area. Its county seat is Towson. The name of the county was derived from the barony of the Proprietor of the Maryland colony, Cæcilius Calvert, 2nd Baron Baltimore, in County Longford, Ireland. Baltimore County does not actually include the city of Baltimore, which left the county to become an independent city in 1851. The northern regions of Baltimore County are primarily rural, featuring a "Piedmont Plateau" landscape of rolling hills and deciduous forests. The southern and south-central regions of the county lie on the Atlantic coastal plain are primarily suburban in character. Among the county's major employers are the Social Security Administration, which has its national headquarters in Woodlawn, and Black & Decker in Towson. During World War II, the Glenn L. Martin Company in MiddleRiver had 53,000 employees manufacturing airplanes for the war effort and Bethlehem Steel had more than 30,000 workers at its sprawling Sparrows Point steel mill.