Willis Tower, formally known and still commonly referred to as Sears Tower, is the tallest building in the United States, reaching a staggering 108 floors. Your visit to Chicago is not complete without a trip up to the Tower's observation deck, called the Skydeck, to get the absolute best view that stretches for miles of Chicago. The Skydeck is unique in that visitors step onto a retractable glass balcony extending a few feet from the building, and are able to see Chicago from all angles, including a birds-eye view via the glass floor.