Chicago, officially the City of Chicago, is the most populous city in the U.S. state of [[Illinois]], and the third most populous city in the United States, following [[New York City]] and [[Los Angeles]]. Chicago is the county seat of Cook County, the second most populous county in the U.S., while a small portion of the city's O'Hare Airport also extends into DuPage County. Located on the shores of freshwater Lake Michigan, 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-19th century. After the Great Chicago Fire of 1871, which destroyed several square miles and left more than 100,000 homeless, the city rebuilt.