Tallest building in the world for over 20 years, this 110-story feat of engineering cuts an unmistakable profile on the city's skyline. Though its formal name changed from the Sears Tower to Willis Tower in 2009, the structure still holds the title of world's tallest building to the tips of its antennae, and it remains—so far—the tallest building in the US. It may also be the deepest. The tower is composed of nine rectangular tubes resting on more than 100 steel and concrete caissons anchored into the bedrock hundreds of feet below ground. These 75-foot-high bundled tubes provide maximum resistance to high winds.