Willis Tower

On 2 July 2009, a viewing ledge built of three 1.3-centimeter thick glass layers, and suspended from the 103rd floor, opened to the public.

Until Taipei 101 was topped out in 2004, the Sears Tower held title to the world's highest roof and highest occupied floor.

The two antennae on the roof give the tower the highest "tip height" of any skyscraper in the world.

The height was listed in many sources for years as 1,454 feet. This is the result of misprinted data which was copied by several sources until it was corrected by the engineers in the middle 1990s.

The lobby contains the moving sculpture "The Universe" by Alexander Calder.

The structure is formed from 9 bundled square tubes, each 75 feet wide with no columns between the core and perimeter. Two of the tubes are 50 floors high, two are 66 floors, three are 90, and two are 108.

The floor count is usually given as 110; however this figure includes the elevator box and its roof, features not normally counted as floors.

This has been the tallest building in Chicago since 1974. For a timeline of Chicago's tallest building through history, start with Holy Name Cathedral.

The observatory elevators are among the world's fastest at 1600 feet per minute.

Double-decker express elevators take passengers from the first two floors to skylobbies at floors 33/34 and 66/67.

Twenty-eight acres of black anodized aluminum panels and approximately 16,100 bronze-tinted windows form the tower's facade.

The lobby floor is decorated with metal tiles in a stylized design based on the bundled tube structure.

Sears Tower was topped out on May 3, 1973, surpassing One World Trade Center as the world's tallest building.

This is not the original Sears Tower; the previous headquarters of Sears Roebuck was a vast building on Chicago's West Side that included the Sears Merchandise Building Tower.

A renovation in the 1980s added a barrel-vaulted entrance pavilion to the west side and a small cut-away atrium inside the east entrance.

Sears Roebuck & Co. originally occupied the lower half of its namesake tower. In 1995 the company moved to a low-rise complex in the northwest Chicago suburb of Hoffman Estates.

Sears executives chose to build a single tall headquarters building over an alternative proposal for twin towers at half the existing tower's height.

The Skydeck occupies the 103rd floor, the highest non-mechanical floor in the building. It has its own entrance on Jackson Boulevard, and attracts approximately 1.5 million visitors per year.

Before the tower was developed, its site was split in half by West Quincy Street. Sears paid the city of Chicago $2.7 million for the street segment.

The building's facade has been climbed twice: by Dan Goodwin in 1981 and by Alain Robert in 1999.

2.5 million cubic feet of concrete were used during construction.

In 2000, four high-definition television antennas were lifted to the roof by helicopter.

The tower's sway has been known to induce motion sickness in people working at the top.

The tower has a pressure lock (like an airlock) at the freight entrance to combat the 'stack effect' generated by the differential in air pressure caused by cold air meeting warm air in the building.

World's tallest building from 1974 - 1996; surpassed by Petronas Tower 1 in Kuala Lumpur, Malaysia.