New Haven is the second-largest municipality in Connecticut and the sixth-largest municipality in New England with a core population of about 124,000 people. "New Haven" may also refer to the wider Greater New Haven area, which has nearly 600,000 inhabitants in the immediate area. It is located in New Haven County, on New Haven Harbor, on the northern shore of Long Island Sound. One year after its founding in 1638, eight streets were laid out in a grid of four streets by four streets creating what is now commonly known as the "Nine Square Plan", which is recognized by the American Institute of Certified Planners as a National Historic Planning Landmark. The central common block is New Haven Green a 16-acre (6 ha) square, now a National Historic Landmark and the center of Downtown New Haven. New Haven had the first public tree planting program in America, producing a canopy of mature trees (including some large elms) that gave New Haven the nickname "The Elm City". The city is the home of Yale University. Along with Yale, health care (hospitals, biotechnology), professional services (legal, architectural, marketing, engineering), financial services, and retail trade form the base of the economy. Since the mid-1990s, the city's downtown area has seen extensive revitalization.