Underflow Tunnel and Reservoir Plan

Chicago, Illinois, United States

Also known as: Deep Tunnel

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July 14, 2026

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The Underflow Tunnel and Reservoir Plan, also known as the Deep Tunnel, is an ambitious project in Chicago, Illinois, designed to combat both pollution and flooding. Chicago's combined sewer system, which carries both storm runoff and sanitary sewage, is overwhelmed by severe storms, leading to flooding and the discharge of untreated sewage into Lake Michigan. This plan involves a vast network of tunnels deep beneath the city to collect and store stormwater. This stored water can then be gradually treated by existing sewage plants. The project also aims to reduce the need for surface quarrying by utilizing mined limestone for construction aggregate. The reversal of the Chicago River's flow in 1900 was a precursor to managing the city's water and waste challenges.123

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Chicago
Municipal services
…an ambitious project popularly called Deep Tunnel. It consists primarily of a vast system of large tunnels bored in the bedrock deep beneath the region that collects and stores stormwater until it can be processed at treatment facilities.
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…an ambitious project popularly called Deep Tunnel. It consists primarily of a vast system of large tunnels bored in the bedrock deep beneath the region that collects and stores stormwater until it can be processed at treatment facilities.
Read More

tunnels and underground excavations
Potential applications
…an imaginative concept is Chicago’s Underflow Tunnel and Reservoir Plan, which is intended to alleviate both pollution and flooding. Like most older cities, Chicago has a combined sewer system that carries both storm runoff and sanitary sewage during wet weather but only sanitary sewage during dry weather. The city’s huge…
Read More
…an imaginative concept is Chicago’s Underflow Tunnel and Reservoir Plan, which is intended to alleviate both pollution and flooding. Like most older cities, Chicago has a combined sewer system that carries both storm runoff and sanitary sewage during wet weather but only sanitary sewage during dry weather. The city’s huge…
Read More