The Mile-High Illinois, Illinois Sky City, or simply The Illinois is a visionary skyscraper that is proposed to be over high, conceived and described by American architect Frank Lloyd Wright in his 1957 book, A Testament. The design, intended to be built in Chicago, included 528 stories, with a gross area of . Wright stated that there would be parking for 15,000 cars and 100 helicopters. If built, it would top the list of the tallest buildings in the world by far, being more than four times the height of the Empire State Building, almost twice as tall as the world's current tallest building, the Burj Khalifa; and about taller than the proposed Jeddah Towerthe latter two both designed by Adrian Smith, whose designs for both buildings are said to have been inspired by that of The Illinois. Technical feasibility Wright believed that it would have been technically possible to construct such a building even at the time it was proposed. At the time, the tallest skyscraper in the world was New York's Empire State Building, at less than a quarter of the height suggested for the Illinois. It probably would have been possible to erect a self-supporting steel structure of the required height, but there are a number of problems that occur when a building is that tall. The material used to build tall towers at the time, steel, is quite flexible. This allows towers to sway substantially in the wind, causing discomfort for occupants of the higher floors. Though Wright acknowledged this problem in his original proposal, he believed the tripod design of this tower (similar to that of the CN Tower, which was not designed until a decade after Wright's death) combined with its tensioned steel frame and the integral character of its structural components would counteract any oscillation. This also could have been solved by placing a tuned mass damper somewhere within the tower as was done in the Citigroup Center and Taipei 101, although this design innovation was not well-known until decades later. The late 1990s and early 2000s saw substantial increases in the load-bearing strength of concrete, making it possible to build entirely in this stiff material. Wright explained that there would be 76 elevators, each having five-floor-high tandem cabs, serving blocks of five floors simultaneously. The 76 elevators would be divided into five banks or groups, with each elevator group serving a hundred-floor