The Tunnel and Reservoir Plan (TARP), also known as "The Deep Tunnel," is the Metropolitan Water Reclamation District’s (MWRD) large scale engineering project designed to reduce flooding and combined sewer overflows (CSOs) within the MWRD’s service area including the Chicago River watershed in Cook County, an area that includes Chicago and dozens of other communities.

TARP’s primary purpose is to reduce CSOs, basement flooding, and backflows into Lake Michigan by using a combination of tunnels and reservoirs to store combined stormwater and sewage after heavy rainfalls until it can be treated, rather than discharging the polluted mess into local waterways.

Approved as the regional plan to address these issues in 1972, construction started in 1975 and was scheduled for completion in 1984. As of 2020, 109 miles of tunnels are complete as is the Gloria Majewski (formerly O’Hare) Reservoir, the Thornton Reservoir, and Phase I of the McCook Reservoir. Phase II of the McCook Reservoir is scheduled to be completed in 2029. The total capacity will be 17 billion gallons when complete.

A combined sewer overflow (CSO) is the discharge of sewage and industrial discharges mixed stormwater directly into a waterway.