Baltimore County is the third-most populous county in the U.S. state of Maryland and is part of the Baltimore metropolitan area. Baltimore County (which partially surrounds, though does not include, the independent City of Baltimore) is part of the Northeast megalopolis, which stretches from Northern Virginia northward to Boston. Baltimore County hosts a diversified economy, with particular emphasis on education, government, and health care. Much of Baltimore County is suburban, straddling the border between the Piedmont plateau to the northwest and in the southern and southeastern regions of the county bordering the Patapsco River and the Chesapeake Bay, the Atlantic coastal plain. Northern Baltimore County is primarily rural, with a landscape of rolling hills and forests, and shares the geography with its neighbors to the east and west, Carroll County and Harford County.