The 1451ft Willis Tower (formerly the Sears Tower), was the tallest building in the world until 1998, when Malaysia’s Petronas Towers nudged it from the top by the length of an antenna; both have since been eclipsed by further construction projects in Taiwan and the Middle East. Various companies occupy the tower (Sears moved out in the early 1990s), and it’s so huge that it has more than one hundred elevators. Two ascend, in little more than a minute, from the ground-level shopping mall to the 103rd-floor Skydeck Observatory for breathtaking views that on a clear day take in four states – Illinois, Michigan, Wisconsin and Indiana. Visitors can also peer down onto the city from one of the elaborate glass boxes that hang off the side of the Skydeck. Look east for the distinctive triangular Metropolitan Detention Center, where prisoners exercise on the grassy roof beneath wire netting to ensure they don’t get whisked away by helicopter.