Tallest building in Chicago, Illinois — For 20 years, the Sears Tower held the record for the world's tallest building. Completed in 1973, the 110 stories high building is almost half a kilometre tall. And it still holds the record for having the world's highest antenna. John Zils, structural engineer and designer of this mega-structure, shares some of the tower's secrets. And get a behind-the-scenes look at some of the various systems and controls that are vital to the building, such as security monitoring, elevators, the water and power distribution that supplies its many floors, and even the machines that clean the windows.