On April 13, 1992, water tore a 20-foot long hole through the wall of a tunnel 20 feet below the bed of the Chicago River, some 50 feet below downtown Chicago. Over 200 million gallons of water surged through an extensive series of underground tunnels, affecting over 30 major buildings including City Hall and the financial markets. Lower levels of major office high-rises held up to 40 feet of water, and the city center was evacuated out of fear that electrical or utility connection failures could endanger lives.
Nearly a century earlier, a commercial system of tunnels had been constructed that connected major Chicago buildings to an underground freight service. A railway transported coal into and ashes from the buildings’ boiler rooms while additional tunnel connections served major railroad and port facilities. Ultimately, the system covered every block downtown—a network over 50 miles long.
Eleven tunnels passed just below the bottom of the Chicago River. At each end water-tight doors could be closed in case of leaks, and individual buildings also installed doors to prevent flooding. In 1959, after a series of financial setbacks, the system was abandoned and fell into disrepair. Bulkhead doors became non-operational, and building flood doors were often permanently opened to provide cool air from the tunnel.
In 1991, workers replacing wooden pile clusters at a bridge pier apparently had no knowledge of the existing tunnel system beneath them. Pressure from soil displaced by pile driving collapsed the concrete tunnel wall inwards, exposing clay and new wood piles and allowing mud and water to seep slowly into the tunnel. Many months after the initial damage, the breach abruptly destabilized, allowing the river to flood downtown Chicago and causing estimated damages of $2 billion.
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