Willis Tower

Chicago has long been considered an architectural mecca. By leveling the heart of the city's downtown, the Chicago Fire of 1871 actually provided a blank canvas for renowned architects such as Louis Sullivan, Daniel Burnham, and William Le Baron Jenney. Wood-frame construction gave way to the use of fire-resistant iron and steel, which, in turn, would lead to the construction of the world's first skyscraper. As a groundbreaker of innovative architecture, the city fosters and supports creative building and engineering feats. Soaring 1,450 feet into the sky, the Willis Tower, seen here, was built with a tubular frame, a concept by which the load is carried by exterior walls of rectangular tubes. Designed by the architectural firm of Skidmore, Owings & Merrill and completed in 1973, Willis Tower held its distinction of being the world's tallest building until 2004. This particular image was taken at dusk. Interestingly, as the sun drops over the horizon, those working on the lower floors have already experienced the sunset and their offices are lit; whereas, because of the tower's height, the occupants on the upper floors are still reaping the benefits of the last light of day.