Aon 01

The Aon Center is currently Chicago's third tallest building, and the seventh tallest in North America. Opened as the Standard Oil Building when it was completed in 1973, "Big Stan" was the tallest building in the city until the Sears Tower opened a year later, and at the time it was the tallest building in the world to be clad in marble. Unfortunately the Italian Carrara marble (the same used for Michelangelo's David) was too thin and fragile for Chicago's extreme temperatures and began to crack and fall off. It was then replaced with granite from North Carolina. The building's exterior and engineering structure is very similar to the original World Trade Center Twin Towers in New York and gives it prominence in Chicago's skyline along Lake Michigan. The building has made appearances in numerous film and TV, but was a primary location in Transformers: Dark of the Moon. It became known as Aon Center in 2001, and was recently sold for an estimated $712 million. Rumors of a new public observation deck are circulating on one of the tower's top floors.