Chicago is the third largest city in the United States and the heart of a metropolitan area of over 8 million people, it is the commercial, financial, industrial, and cultural center for a vast region and a mid-continental shipping point. A major Great Lakes port, it is also an historic rail and highway hub. O'Hare International Airport is the second busiest in the nation. An enormous variety of goods are manufactured in the area. Despite an overall decline in industry, Chicago has retained large grain mills and elevators, iron- and steelworks, steel fabricators, and meatpacking, food-processing, chemical, machinery, and electronics plants. The city has long been a publishing center; the Chicago Tribune is among the most widely read newspapers in the country.     Chicago covers over 200 sq mi (520 sq km); it extends more than 20 mi (32 km) along the lakefront, then sprawls inland to the west. Its metropolitan area stretches in the north to the Wisconsin border and in the south to industrial suburbs on and beyond the Indiana border. In addition to its noted expressways and boulevards, Chicago has a system of elevated (partly underground) railways that extend into the heart of the city, making a huge rectangle, the celebrated Loop, which gives its name to the downtown section.