Chicago has a problem with water. Always has. Back in the 1800s, the city literally jacked up its buildings with screws to get out of the mud, and later, they famously pulled off the engineering miracle of reversing the flow of the Chicago River. But even that wasn't enough to stop the basement-flooding, sewage-overflowing mess that happens every time a heavy summer storm rolls through the Midwest. That is where the Chicago Deep Tunnel Project comes in. Officially called the Tunnel and Reservoir Plan (TARP), it is one of those massive, invisible infrastructure projects that sounds like science fiction. Imagine a network of tunnels, some 33 feet wide, sitting 350 feet below your feet, stretching for over 100 miles. It’s basically a second city for water.
It started in the 70s. People were tired of "combined sewer overflows" (CSOs). See, in old cities like Chicago, the pipes that carry rainwater and the pipes that carry toilet water are the same pipes. When it pours, the system gets overwhelmed. To prevent the mix from backing up into your kitchen sink, the city used to just dump the raw sewage straight into the river and Lake Michigan. Gross, right? TARP was designed to catch that overflow, hold it in deep rock tunnels, and then pump it back to treatment plants once the rain stops. It’s a massive, multi-decade bet on civil engineering.
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The Massive Scale of the Chicago Deep Tunnel Project
If you actually saw the scale of this thing, you’d probably get vertigo. We are talking about 109 miles of tunnels bored through solid Niagaran dolomite limestone. To get a sense of how big 33 feet in diameter is, imagine a three-story house fitting comfortably inside the tunnel with room to spare. The Metropolitan Water Reclamation District of Greater Chicago (MWRD) didn't just stop at tunnels, though. The project also includes massive reservoirs like the Thornton Composite Reservoir and the McCook Reservoir.
Thornton is wild. It’s a repurposed limestone quarry. It can hold nearly 8 billion gallons of water. When it’s empty, it looks like a canyon out of a Western movie. When it's full, it's the only thing keeping the south side of Chicago from becoming an accidental lake. The McCook Reservoir, which is still being finished in phases, will eventually hold another 10 billion gallons. You can’t even wrap your head around that much liquid. It’s enough to fill about 15,000 Olympic-sized swimming pools.
Why digging through rock is harder than it looks
You’d think digging a hole is simple. It isn't. The engineers had to use massive Tunnel Boring Machines (TBMs) that grind through the rock like giant mechanical worms. These things are hundreds of feet long. They leave behind a smooth, circular wall that is then often lined with concrete to prevent leaks. The geology of Chicago is weirdly perfect for this because the limestone is sturdy enough to hold the weight of the city above without collapsing, but it’s still a nightmare to move that much debris to the surface.
The $4 Billion Question: Does it actually work?
Critics have been yelling about the cost for decades. Honestly, they have a point. The Chicago Deep Tunnel Project was originally estimated to cost much less than the billions it has eventually swallowed. Construction has spanned over 50 years. Some people born when the first shovel hit the dirt are now retired, and the project still isn't technically 100% finished.
But does it work? Mostly, yeah.
Since the tunnels and the first phases of the reservoirs came online, the number of times raw sewage hits the lake has dropped off a cliff. Before TARP, the river would be reversed back into Lake Michigan (the city's drinking water source) several times a year during storms. Now, it happens maybe once every few years during "once-in-a-century" weather events. It has saved billions in property damage. If you've ever had a flooded basement, you know that "billions" isn't just a statistic; it’s a personal tragedy.
The climate change curveball
Here is the kicker. The engineers in the 1970s were geniuses, but they weren't psychic. They designed the Chicago Deep Tunnel Project based on rainfall patterns from the mid-20th century. But 2026 isn't the 1970s. We get these "rain bombs" now—super-intense bursts of five inches of rain in two hours. Even a 30-foot tunnel can’t swallow that much water that fast.
This is why some people argue that TARP is a "gray infrastructure" relic. They say we should have spent the money on "green infrastructure"—like permeable pavement, rain gardens, and green roofs that soak up water where it falls. The reality is probably that we need both. You can't plant enough flowers to soak up 18 billion gallons of overflow, but you also can't just keep digging deeper holes forever.
What Most People Get Wrong About TARP
A common myth is that the tunnel is just a giant sewer. It's not. It's a holding tank. The water in there is "pre-treatment." It sits there until the Stickney Water Reclamation Plant (one of the largest in the world) has the capacity to clean it. If the tunnel stays full because it's raining for three days straight, the system still fails. That’s the bottleneck. You can have the biggest tunnel in the world, but if the treatment plant is at max capacity, the water has nowhere to go but up.
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Another thing? People think the project is a failure because flooding still happens. But that's usually because the local neighborhood sewers—the small pipes under your street—are too small or clogged. The "Deep Tunnel" is like a massive highway. If the on-ramp (your street drain) is blocked, it doesn't matter how wide the highway is.
The Future of the Deep Tunnel
The final stage of the McCook Reservoir is slated for completion around 2029. Once that’s done, the system will be at its full theoretical capacity. Engineers are already looking at ways to use AI and real-time sensors to "gate" the water flow, basically playing a giant game of Tetris with sewage to make sure no single part of the system overflows before another.
It is a testament to human ego, honestly. We decided to build a city on a swamp, and when the swamp tried to take it back, we just spent 50 years and $4 billion digging a bigger swamp underneath the first one.
Actionable Insights for Homeowners and Residents
If you live in the Chicagoland area, don't assume the Chicago Deep Tunnel Project makes you immune to flooding. You have to take your own steps because the "last mile" of infrastructure is your responsibility.
- Install an overhead sewer or backwater valve. This is the only way to truly stop the city's main line from pushing sewage into your basement when the system is under pressure.
- Disconnect your downspouts. If your gutters go straight into the sewer, you are part of the problem. Divert that water to your lawn or a rain barrel.
- Check the MWRD "Overflow Action" alerts. When a big storm is coming, the MWRD literally asks people to delay showers and laundry. It sounds silly, but if 10 million people wait two hours to flush, it keeps millions of gallons out of the tunnels, leaving room for the rain.
- Support green infrastructure at the local level. Encourage your ward or suburb to invest in bioswales and permeable alleys. The "Deep Tunnel" is the last line of defense, but we need better front-line blockers.
The Chicago Deep Tunnel Project is a marvel of technology, but it’s a reminder that we are always in a tug-of-war with nature. We might be winning for now, but nature has a lot of patience and a lot of water.