Chicago O'Hare Airport, Illinois

O'Hare, Chicago's main airport, is usually cited as one of the busiest airports in the world, with over 2,000 take-offs and landings a day. Like most major airports in the country, it started out as a military base in WWII, the site of a Douglas Aircraft Plant making military transport planes. Known after the war as Air Force Plant 8, the assembly buildings covered around 2 million square feet. The airport reverted to civilian use in the 1950s, and most of the Douglas Aircraft buildings, located at what is now the northeast corner of the airport (at Manheim and Higgins), were removed, and the site redeveloped, as part of the much expanded airport of today.