The former Amaco or Standard Oil Building, now Aon Center, this building is a square box a la WTC. Built in 1973, over 203 smoots high, it has 83 floors. Was the tallest in Chicago for a year until Sears. In 1974 a slab of marble came off the building and plunged through the roof of a building next door. From 1989-1992 the marble panels were replaced with granite, a job costing $80,000,000.

According to Emporis, it is the tallest building in the world without antennas, spires or finials.