Chicago is the third most populous city in the United States, after New York City and Los Angeles. With over 2.7 million residents, it is also the most populous city in both 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 United States. It is the birthplace of the skyscraper and perhaps the most influential architectural city of the 20th century. Chicago saw the creation of the first standardized futures contracts at the Chicago Board of Trade; today its successor has evolved into the largest and most diverse derivatives market in the world, generating 20% of all volume in commodities and financial futures. 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. After the Great Chicago Fire of 1871, which destroyed several square miles and left more than 100,000 homeless, the city made a concerted effort to rebuild. The construction boom accelerated population growth throughout the following decades, and by 1900 Chicago was one of the five largest cities in the world. During this period, Chicago made noted contributions to urban planning and zoning standards, new construction styles (including the Chicago School of architecture), the development of the City Beautiful Movement, and the steel-framed skyscraper. Positioned along Lake Michigan, the city is an international hub for finance, commerce, industry, technology, telecommunications, and transportation. O'Hare International Airport is the one of the busiest airports in the world, and the region also has the largest number of U.S. highways and railroad freight. In 2012, Chicago was listed as an alpha global city by the Globalization and World Cities Research Network, and it ranked seventh in the entire world in the 2017 Global Cities Index. Chicago has the fourth-largest gross metropolitan product in the world—about $670.5 billion according to September 2017 estimates-ranking it after the metropolitan areas of Tokyo, New York City, and Los Angeles, and ranking ahead of number five London and number six Paris. Chicago is also the largest economy in the Midwestern United States. The city has one of the world's largest and most diversified and balanced economies; not dependent on any one industry, with no single industry employing more than 14% of the workforce. Chicago has professional sports teams in each of the major professional leagues. The city has many nicknames, the best known being the Windy City and Chi-Town.