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FILE - This Aug. 2, 2005 file photo shows Aon Corporation's worldwide headquarters in downtown Chicago. At 1,136 feet, it ranks as one of the tallest buildings in the country. One World Trade Center, the giant monolith being built to replace the twin towers destroyed in the Sept. 11 attacks will exceed the Aon Center’s height and lay claim to the title of New York City’s tallest skyscraper on Monday, April 30, 2012, as workers erect steel columns that will make its unfinished skeleton a little over 1,250 feet, just high enough to peak over the observation deck on the Empire State Building. (AP Photo/Charles Rex Arbogast, File)

FILE - This Aug. 2, 2005 file photo shows Aon Corporation's worldwide headquarters in downtown Chicago. At 1,136 feet, it ranks as one of the tallest buildings in the country. One World Trade Center, the giant monolith being built to replace the twin towers destroyed in the Sept. 11 attacks will exceed the Aon Center’s height and lay claim to the title of New York City’s tallest skyscraper on Monday, April 30, 2012, as workers erect steel columns that will make its unfinished skeleton a little over 1,250 feet, just high enough to peak over the observation deck on the Empire State Building. (AP Photo/Charles Rex Arbogast, File)

World's tallest buildings

The world's tallest tower and Japan's biggest new landmark, the Tokyo Skytree, opened to the public on May 22. Dubai's Burj Khalifa holds the title of world's tallest structure, which stands 828 meters (2,717 feet).