The Willis Tower (formerly known as the Sears Tower) is located in downtown Chicago at 233 S Wacker Drive, and is one of the tallest buildings in the world. It was designed in the modernist style by the architects Skidmore, Owings & Merrill (SOM) and was completed in 1974.

The building's exterior is characterized by its sleek, minimalist design, with a rectangular shape and a series of setbacks that help to reduce wind resistance and increase stability. The exterior is clad in black aluminum and bronze-tinted glass, giving the building a distinctive appearance that stands out against the Chicago skyline.

The Willis Tower's most notable feature is its observation deck, known as the Skydeck, which is located on the 103rd floor and offers stunning views of the city and Lake Michigan. The Skydeck is supported by four steel columns that extend down to the foundation, giving visitors the sensation of floating above the city.

The Willis Tower also features a number of innovative engineering and design features, including a bundled-tube structural system that distributes the building's weight evenly and helps to reduce wind sway. The building was also the first to use a system of sky lobbies, which are large transfer floors that allow passengers to switch elevators without having to return to the ground floor.

The Willis Tower is a striking example of modernist architecture and an important landmark in the city of Chicago. Its innovative design, engineering features, and stunning views from the Skydeck make it a popular destination for visitors and residents alike.