Willis Tower (Formerly the Sears Tower)
Think of it as the world’s tallest recycling effort. Or maybe it’s just an exceptionally ambitious retrofit. In any case, the record-breaking Sears Tower, completed in 1973—it reigned as the world’s tallest until the Petronas Towers in Kuala Lumpur topped it in 1998—is about to become more energy-efficient. The building’s current owner, which is no longer Sears, has retained architecture firm Adrian Smith + Gordon Gill (Smith, formerly of Skidmore, Owings & Merrill, designed Burj Dubai), who will explore how the building’s lighting, mechanical systems, elevators, and glass skin can be updated to bring the building up to LEED standards. While there’s an endless parade of new green skyscrapers planned for cities around the country (including Chicago’s Smith + Gill–designed, turbine-equipped “Clean Technology Tower”), the amount of energy new towers will save is minuscule compared with what can be achieved by overhauling existing buildings. From the 103rd floor of the tallest building in North America, Chicago's message is clear: Think big.