Head east on Madison to Millennium Park, and you'll find the Aon Center (formerly the Amoco Building, and the Standard Oil Building before that) rising up behind it. Completed in 1973, the Aon Center is Chicago's third tallest building in Chicago. It was bumped out of its second place spot behind the Sears Tower in 2008 after the Trump Tower was built.
The construction process used to build the Aon Center was the same as the World Trade Center towers in New York City, and it has the same plain square shape. The sides of the building were originally clad in Italian Carrara marble, but designers used slabs that were too thin which proved to be an incredibly costly mistake -- after years of stopgap measures, the owners of the building had to finally reface the entire building with granite instead to the tune of $80 million.