Chicago's Willis Tower, formerly known as Sears Tower, was the world's tallest building from 1974 to 1998 and remains the tallest building in the U.S. to this day. Its 1451-foot (442 m) height adds up to enough window area to keep a window washer busy for life, or space for enough solar panels to be comparable to a 10 acre solar power plant. As part of a pilot project, the south facing windows of the 56th floor of the Willis Tower will be replaced with Pythagoras Solar's transparent solar windows which cut down on heat gain – and therefore cooling costs – as well as harnessing energy from the sun.
If you're ever likely to suffer from Acrophobia (from the Greek: akron , meaning "peak, summit, edge" and phobos, "fear" and meaning an extreme or irrational fear of heights), then Sears Tower in Chicago is the place that'll surely bring it on. Standing at 1,450 feet and 110 stories, Sears Tower is the tallest building in North America and the third tallest building in the world. The building recently added a new attraction as part of an updated Skydeck Chicago experience. Christened “The Ledge”, it's a series of glass bays on the 103rd floor that offer visitors unobstructed views of Chicago – 1,353 feet straight down.