CHICAGO, IL - MARCH 04: The Willis Tower, formerly known as the Sears Tower, dominates the southern end of the downtown skyline on March 4, 2015 in Chicago, Illinois. The building, completed in 1973, was the world's tallest for more than two decades. It is reported to be up for sale with an asking price of $1.5 billion. (Photo by Scott Olson/Getty Images)
