The Deep Tunnel

In the early 1970s, raw sewage ended up in local rivers on an average of once every four days. Urged by Mayor Richard J. Daley, the Sanitary District developed a control system called the Tunnel and Reservoir Plan (TARP), nicknamed Deep Tunnel.

By decreasing the number of times raw sewage ends up in the river, TARP helps clean up local waterways. During times of heavy rains, the combined sewer diverts stormwater and sewage into underground tunnels and reservoirs, which hold the wastewater until the treatment plants can catch up and process the excess.

TARP was a massive undertaking. The Mainstream Tunnel, which runs parallel to the Chicago River, is thirty-five feet in diameter, holds one billion gallons, and is carved in the limestone bedrock approximately 300 feet below ground.