Chicago (/ʃᵻˈkɑːɡoʊ/ or /ʃᵻˈkɔːɡoʊ/), officially the City of Chicago, is the third-most populous city in the United States. With over 2.7 million residents, it is the most populous city in the state of Illinois, and the Midwestern United States. It is the county seat of Cook County. The Chicago metropolitan area, often referred to as Chicagoland, has nearly 10 million people and is the third-largest in the U.S. Chicago was incorporated as a city in 1837, near a portage between the Great Lakes and the Mississippi River watershed, and grew rapidly in the mid-nineteenth century. Positioned along Lake Michigan, the city is an international hub for finance, commerce, industry, technology, telecommunications, and transportation: O'Hare International Airport is the second-busiest airport in the world when measured by aircraft traffic; the region also has the largest number of U.S. highways and rail road freight. In 2012, Chicago was listed as an alpha global city by the Globalization and World Cities Research Network, and ranked seventh in the world in the 2014 Global Cities Index. Chicago has the third-largest gross metropolitan product in the United States—about $640 billion according to 2015 estimates. The city has one of the world's largest and most diversified economies with no single industry employing more than 14% of the workforce. In 2016, Chicago hosted over 54 million domestic and international visitors, a new record making it one of the top visited cities in the nation. Landmarks in the city include Millennium Park, Navy Pier, the Magnificent Mile, Art Institute of Chicago, Museum Campus, the Willis (Sears) Tower, Museum of Science and Industry, and Lincoln Park Zoo. Chicago's culture includes the visual arts, novels, film, theater, especially improvisational comedy, and music, particularly jazz, blues, soul, hip hop, gospel and house music. There are many colleges and universities in the Chicago area; among these, Northwestern University, University of Chicago, and the University of Illinois at Chicago are classified as "highest research" doctoral universities. Chicago also has professional sports teams in each of the major professional leagues. The city has many nicknames, the best-known being the Windy City.