In 1972, Chicago began digging 110 miles of sewer tunnel up to 300 feet down; 50 years on, the Deep Tunnel has captured more than a trillion gallons of combined sewage

Somewhere around 300 feet underneath the city of Chicago, exists a network of tunnels so large that it could make even a James Bond villain jealous. It is not an expansion of the city’s subway system or some sort of underground bunker but a sewer: a massive 110-mile-long sewer system built in the course of many decades, which has saved more than a trillion gallons of untreated sewage and stormwater from flowing out into Lake Michigan. As per the Metropolitan Water Reclamation District of Greater Chicago, the Tunnel and Reservoir Plan (TARP) or Deep Tunnel project, as it is often referred to, celebrated its 50-year anniversary back in 2022, and the tunnels and reservoirs have continued adding to that trillion-gallon total in the years since.

The gross problem hiding under half of urban America

And here’s the part most people tend to gloss over: it is not an anomaly. There are somewhere around 700 other cities in America that still operate under what’s called a combined sewer system, in which residential sewage and storm runoff water travel through the same pipes. As highlighted by the US Government Accountability Office in a 2023 report, heavy rain can cause these pipes to overflow, sending untreated runoff directly into rivers, lakes, and streams. This was business-as-usual well before any concern arose over microplastics or PFAS.

Why Chicago went full "Mole Man" on this problem

By the 1960s, Chicago-area sewers were overflowing into local waterways more than 100 days a year, and flooding had become a persistent, region-wide problem, according to the Metropolitan Water Reclamation District of Greater Chicago. Since they knew that they would not have time to rebuild a whole metro area from the ground up, in 1972, city officials approved the construction of something close to preposterous in scale: four tunnels, built 150 to 300 feet below the ground, meant to intercept the excess runoff and store it until it could be processed in treatment facilities. According to the MWRD report, the tunnel portion alone was completed in 2006, reducing sewer pollution by 85 percent. The reservoir portion of the project, which includes the colossal McCook Reservoir, is still underway; MWRD's most recent public update (2026) now puts the completion of the final stage at the end of 2032, pushed back from an earlier 2029 target due to slower-than-expected stone mining at the reservoir site.

However, this problem isn’t exclusive to Chicago. Combined sewer overflows have posed serious public health risks, and not just theoretical engineering problems. As described in a research article titled “A Review of Combined Sewer Overflows and Gastrointestinal Illness in a Changing Climate,” published in Current Epidemiology Reports, sewer flooding results in the discharge of untreated sewage containing pathogens into the waterways used for recreational activities such as swimming, kayaking, and fishing. The researchers found that CSOs have been associated with higher rates of gastrointestinal illness in affected areas.

Rain is not acting like it normally should, and and that mismatch is straining a system built for a different climate. Increased rains and heavier rains mean that the very system that was developed decades agohas, in some storms, already begun exceeding its current capacity, a gap the still-unfinished McCook Reservoir is specifically meant to close once complete. As pointed out by GAO, EPA has already required 700 municipalities to solve their overflow problem starting from 1994 but has not been able to define clear objectives and track how well things have progressed.

So is the Deep Tunnel actually working?

For the most part, yes, and that deserves some recognition. The completed 110-mile tunnel network, that is, the "Deep Tunnel" proper, has already cut sewer overflow pollution by 85 percent since 2006, and the wider TARP system it belongs to has captured more than a trillion gallons of combined sewage over the project's history. At the same time, the fact that the reservoirs meant to close the remaining gap won't be finished until 2032 shows that the job isn't done: this is a fifty-year engineering achievement that has protected a Great Lake, but one whose final chapter, and its ability to keep pace with heavier modern storms, is still being written. The fact that it has taken over 50 years to construct a sewage system for one city in the US is either an impressive undertaking or an indictment of the slow handling of mundane public services.