The third-most populated city in the US after New York City and Los Angeles is Chicago, which is also the most populous city in the U.S. state of Illinois. It is also the most populated city in the Midwest, with 2,746,388 residents according to the 2020 census. The city serves as the administrative centre for Cook County, the second-most populous county in the United States, and one of the largest metropolitan areas in the world.
Chicago was established as a city in 1837 and is located on the shore of Lake Michigan, close to a portage between the Great Lakes and the Mississippi River watershed. By 1860, Chicago was the youngest American city to have a population of over 100,000. It had fast growth in the middle of the 19th century. Several square miles of Chicago were devastated by the Great Chicago Fire in 1871, leaving more than 100,000 homeless, but the city's population continued to rise, reaching 503,000 by 1880 and then more than doubling to more than a million within the same decade. In the decades that followed, the construction boom spurred population expansion, and by 1900, less than 30 years after the fire, Chicago had risen to become the fifth-largest city in the world. Chicago made significant contributions to zoning regulations, new building types, and urban planning, including the birth of the City Beautiful Movement and the steel-framed skyscraper.