Built in: Completed in 1974 for Sears, Roebuck & Co's office space.
Built by: Architect Bruce Graham and structural engineer Fazlur Rahman Khan of Skidmore, Owings & Merrill designed the building to be one of the largest office buildings in the world.
Why it's one of the seven wonders: The 1,450ft Willis Tower (still lovingly known by some as Sears Tower) might not be THE tallest skyscraper in the nation anymore thanks to One World Trade Center's ridiculous spire, but it still reigns supreme in Chicago's skyline. Four states are visible from the top of the 110-story tower (Illinois, Indiana, Michigan, and Wisconsin), and the elevators are among the fastest in the world operating at 1,600ft per minute. The building's innovative bundled tube structure (it was made up of nine square "tubes," each essentially a separate building) later influenced the design of other skyscrapers, including the world's current tallest building -- the Burj Khalifa.