Chicago Tunnel and Reservoir Plan (1975–present)
Chicago began digging the Tunnel and Reservoir Plan, known as Deep Tunnel, in the 1970s to capture combined sewer overflows polluting the Chicago River and Lake Michigan. It grew into one of the largest civil works projects in the world, with over 100 miles of deep tunnels.
The earliest tunnel segments began storing overflow within years, measurably cutting pollution spikes during storms.
Decades of phased construction and billions in spending show how long CSO storage programs really take to finish.
Chicago's program is the template for Cleveland's: city sewage systems built before modern treatment rely on deep rock tunnels, and the timelines stretch decades.
