(CNN)Here's some background information about Willis Tower, formerly the Sears Tower, located in Chicago.
The visitors' observation deck, called the "Skydeck," is located on the 103rd floor.
The Sears Tower was the world's tallest building until 1996, when it was surpassed by the Petronas Twin Towers.
Some 25,000 people enter the skyscraper every day.
About 1.5 million visitors go up to the Skydeck each year.
Visitors can step out on to "The Ledge," a glass box which extends out 4.3 feet from the Skydeck.
The Skydeck elevator can travel to the Skydeck in one minute, ten seconds.
Structural Height - 1,450 feet or 110 floors
Height with the two antennas - 1,730 feet
Height to Skydeck/103rd floor - 1,354 feet
Lowest level - 43 feet below Franklin St.
Building Size - 4.56 million gross square feet
Weight - 222,500 tons or 445 million pounds
Cost to build - $150 million
Windows - 16,100
Building exterior - black aluminum and bronze-tinted glass
Architectural firm - Skidmore, Owings & Merrill
Architect - Bruce Graham
Structural Engineer - Fazlur Khan
1970 - Construction begins.
May 3, 1973 - Construction is completed, the last floors are "topped out."
1973 - The Sears Tower is open to tenants.
1974 - The Skydeck opens to the public.
1994 - Sears, Roebuck and Co. sells the building to Aldrich, Eastman & Waltch.
1996 - The Petronas Twin Towers in Kuala Lumpur, Malaysia, surpass the Sears Tower in height to become the world's tallest buildings at 1,483 feet each. The Sears Tower becomes the world's third tallest building, at the time (behind Petronas Tower 1 and Petronas Tower 2).
December 3, 1997 - AEW Partners LP sells the building to Canadian real estate firm, TrizecHahn Corp.
August 28, 2003 - MIAC (Metropolitan Insurance and Annuity Company), an affiliate of MetLife, purchases the tower.
March 2009 - The owners of the Sears Tower announce that the building will be renamed Willis Tower in 2009, after new tenant Willis Group Holdings.
July 2009 - Skydeck's attraction, the Ledge, opens on the 103rd floor. It is a series of glass bays, with glass floors, that extend 4.3 feet outwards from the building. Visitors can now look directly down 1,353 feet to the street below.
August 2009 - The Sears Tower is officially renamed Willis Tower.