Willis Tower is a supertall skyscraper in Chicago, the United States. Previously known as Sears Tower, the tower was the tallest building in America before 2014, as of 2018 it's still the nation's tallest building measured to roof. Below are some interesting facts about the Willis Tower. 
1. Name
Willis Tower was previously called Sears Tower, Sears developed the building in 1970s, and had been owning it since its completion. In 1994 Sears sold the building, and the naming rights of Sears expired in 2003. The building was renamed to Willis Tower in 2009, when London-based insurance broker Willis Group Holdings leased a portion of the building and obtained the building's naming rights, which was agreed as a part of the lease. 
Many locals and other American people dislike the name change and continue to call the building Sears Tower. For those dislike the name of Willis Tower, the good thing is that this naming rights is valid for only 15 years, which means the building's name could be changed again after 2024.
2. Height
Willis Tower is 1450 feet (442m) tall by roof height and architectural height, the tower is topped with two antennas, the western antenna brings the total height of tower to 527 m. 
Since its completion in 1973, Willis Tower had been the tallest building in the world for 25 years, until in 1998, when Petronas Towers in Kuala Lumpur surpassed it by architectural height, not everyone admit that, as both the roof height and total height of Petronas Towers are much lower than Willis Tower.
Petronas Towers became the new tallest buildings was because their spires are considered by CTBUH as an integral architectural element of the towers, while the antennas on Willis Tower are not, the tower didn't have the antennas when it was first built in 1973, the antennas were installed in the later in 1982. As of 2018, Willis Tower is the 19th tallest building in the world among all the complete and topped out buildings. In the United States, the title of the country's tallest building was just overtaken by One World Trade Center in 2014, confusingly, this time CTBUH considers the antenna on One World Trade Center as integral part of its architecture element. However, the roof, highest occupied floor and observation deck of Willis Tower is still higher. 
3. Design
Willis Tower was designed by Bruce Graham and Fazlur Rahman Khan of Chicago based architectural firm Skidmore, Owings & Merrill (SOM), Fazlur Rahman Khan invented the bundled tube structure for Willis Tower, it is said Willis Tower's distinctive appearance was inspired by an advertisement for a package of cigarettes. The tower is composed of nine square tube structures of different heights, and clustered in a 3×3 matrix forming a square base with 225-foot (69 m) sides, only two of these nine tubes (the center tube and the middle tube on the west) reach the top of the tower. 
Fazlur Rahman Khan's Bundled Tube System has been also applied on some other skyscrapers in the world, one of the latest skyscraper being developed that uses this system is the Landmark 81 Tower in Ho Chi Minh City.
4. Tourism
The observation deck on the 103rd floor of Willis Tower is the highest observation deck in the United States. Named Skydeck, the observation deck is at 412 meters off the ground, from there visitors can get the impressive aerial views of Chicago, Lake Michigan and most of Illinois, on some clear days, the views of Wisconsin and two states on the east side of Lake Michigan can also be seen. The Skydeck is one of the most popular tourist attractions in Chicago, attracting around 1.3 million tourists annually.
Four glass sky-boxes are installed outside the west side of observation deck, the sky-boxes extend from the facade of the building about 4 feet, overlooking South Wacker Drive. Visitors can stand on the glass floor and look down at the street 1,353 feet below.
5. Popular culture
Willis Tower is one of the most iconic skyscrapers in America, the tower has been featured in numerous films, TV series, video games, etc. Below are some of notable films with Willis Tower featured.
2014 film Divergent, the story takes place in a future Chicago, the water of Lake Michigan is dried up for unknown reason and replaced by weeds, Willis Tower is still there, but is abandoned and decayed.
2011 science fiction film Transformer: Dark of the Moon: The major battle of Autobots and Decepticons takes place in Chicago, among the skyscrapers in the city, Willis Tower is shown many times in the battle.
2014 science fiction film Transformer: Age of Extinction: In the film, Cade, Shane and Tessa try to escape from a ship to the roof of Willis Tower though a series of wire ropes.
2015 science fiction film Jupiter Ascending: In the film Caine Wise and Jupiter Jones are taken away from the Willis Tower by a spaceship.
2014 video game Watch Dogs: Willis Tower is featured in a modified version of Chicago as the tallest building in the game.
6. The renovation plan
In February 2017, the current owner of Willis Tower Blackstone announced that they will refurbish the building. According to their plan, they will construct a new podium for Willis Tower, the podium will house a new shopping mall, and the top of the podium will have an open-air roof garden.
The observatory on 103rd floor will also be refurbished, the observatory is intended to be expanded to 104th floor, becoming a multi-floor observatory. And a new recreational amenity will be added to the observatory, it is said this new amenity is possibly a glass structure outside the facade, something like the glass slide on US Bank Tower in Los Angeles.
The construction work is planned to be complete in 2019.