The sky deck of the Willis Tower in Chicago stands 1,500 feet over the city’s streets and is home to a panoramic vista that extends for miles over Lake Michigan to the east and the farmland of Illinois to the west. While the glass floor of the viewing station can induce a dizzying swoon, the history of the area is even more vertiginous. Some 26,000 years ago this deck would have sat 1,500 feet below the surface of an ice sheet that extended along the northern United States through the entirety of Canada. The highest skyscrapers in Manhattan would have been encased in ice every bit as much as
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