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The Mile High Illinois, Illinois Sky-City, or simply The Illinois was a proposed skyscraper that would have been 1 mile high, described by Frank Lloyd Wright in his 1956 book, A Testament. The design, intended to be built in Chicago, would have included 528 stories, with a gross area of 18,460,000 square feet. Wright stated that there would be parking for 15,000 cars and 150 helicopters.
Had it been built, it would have been the tallest building in the world by far, being more than four times the height of the then tallest building in the world at that time, the Empire State Building in NYC, and it would be nearly twice as tall as the world's current tallest building, the Burj Khalifa in Dubai.
The building is one of the semi-serious buildings by visionary architects meant to be an alternative to the increasing urban sprawl occurring in most cities. None of these has ever been viewed as financially feasible.
The above linked video is a CAD creation of what it would have been like.