Just south of Chicago lies the Thornton Quarry, a limestone excavation that is nearly 500 feet deep and over a mile wide. I-294 runs directly through it, featuring sheer rock walls that plunge hundreds of feet. This site serves as a significant engineering landmark in America, with a history connected to Chicago founder Gurdon Saltonstall Hubbard.

The video explores the evolution of limestone mining, the construction of the Tri-State Tollway, and the Deep Tunnel Project, highlighting how the former quarry became essential to Chicago's flood control system, capable of holding billions of gallons of stormwater. Additionally, it covers its geological history, which dates back 440 million years to a tropical sea and includes a meteorite impact.

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