Willis Tower United States

Legendary

From 1974 to 1998, it was the tallest building in the world at 1,450 feet.

Photo by Daniel Lobo

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On the 103rd floor of this skyscraper, you can step into a glass box that sticks out from the side of the building. The floor, the walls, and the ceiling are all glass.

You're standing 1,353 feet above the street, with nothing under your feet but a clear panel. It's called The Ledge, and it was added in 2009.

The building stands in Chicago and was built for Sears, which at the time was the biggest store company in the United States. It needed one place for thousands of its workers.

At first, Sears planned a building of about 60 or 70 floors. Then it got more ambitious.

As the plans grew taller, the design passed the height of the World Trade Center in New York, which was still being finished. Sears decided it wanted the tallest building on the planet.

The architects were Bruce Graham and engineer Fazlur Rahman Khan. Building something that tall in the "Windy City" was a real challenge.

Khan's solution was the bundled tube. Instead of one big tower, the building is made of nine square tubes bundled together, which helps it stand strong against the wind.

Not all the tubes go to the top. They stop at different heights, which gives the tower its famous stair-step shape.

Construction started in 1970. When the tower was finished in the mid-1970s, it stood 1,450 feet tall and became the tallest building in the world.

It held that title for more than 20 years. The Petronas Towers in Malaysia finally passed it in the late 1990s.

It stayed the tallest building in the Western Hemisphere even longer, until One World Trade Center in New York passed it in 2013.

Sears moved out in the 1990s. In 2009, a company called Willis Group rented space in the building and got the right to rename it, so the Sears Tower became the Willis Tower.

Plenty of people in Chicago still call it the Sears Tower anyway.

Ride the elevator to the Skydeck on the 103rd floor for views across Chicago and, on a clear day, as far as four states. Step out onto The Ledge if you dare and look straight down at the street below. Then look up from the ground outside to see the nine tubes stepping up toward the sky.

Sources: Britannica, Wikipedia, and CBS Chicago.