workers inspecting the deep tunnel in milwaukee

Milwaukee Deep Tunnels

The Deep Tunnels have prevented more than 151.4 billion gallons of pollution from getting into Lake Michigan. Thanks to the tunnels and many other improvements, MMSD has captured and cleaned 98.6% of all the stormwater and wastewater that's entered the regional sewer system since 1994. The national goal is to capture and clean 85% of water for more than 700 cities with systems like ours.

Why Do We Need Deep Tunnels?

The Jones Island and South Shore water reclamation facilities can efficiently clean only a certain amount of water a day (about 150 million gallons on a dry day and 630 million gallons when it rains). When more water gets into the sewers than the water reclamation facilities can handle, you need somewhere to store it so the excess water doesn't cause basement backups or sewer overflows.

Before the Deep Tunnels, the Greater Milwaukee region used to pollute Lake Michigan with an average of 50-60 overflows a year, and now we average 2.2.

combined sewer system graphic of city sewer connecting to deep tunnel

Does the Deep Tunnel System Prevent Sewer Overflows?

Although the deep tunnels can hold 521 million gallons, our climate is changing, and increasingly intense storms and record-setting rainfall in the Greater Milwaukee region make sewer overflows, basement backups, and flooding even greater threats to our community. As a region, we've invested more than $4 billion to reduce sewer overflows. Our current financial forecast through 2033 calls for investing $3.5 billion in clean water infrastructure, flood management, and debt financing to help protect public health and Lake Michigan.

Facts about the Deep Tunnel System

Phase One (1993)

Length: 19.4 miles

Diameter: 17ft. to 32 ft.

Depth Underground: 275 to 340 ft.

Storage: 405 million gallons

Phase Two (2006)

Length: 7.1 miles

Diameter: 20 feet

Depth Underground: 120 - 165 ft.

Storage: 89 million gallons

Phase Three (2010)

Length: 2 miles

Diameter: 21 ft.

Depth Underground: 300 - 325 ft.

Storage: 27 million gallons

Total storage of all three phases: 521 million gallons

Why Do We Have Deep Tunnels?

In 1972, the State of Illinois sued both the Metropolitan Sewerage Commission of Milwaukee County and the City of Milwaukee Commission, a legal battle that went all the way to the U.S. Supreme Court. In 1977, the District reached an agreement with the Wisconsin Department of Natural Resources (DNR) to reduce overflows. The Illinois lawsuit and DNR agreement led to the Water Pollution Abatement Program (WPAP) development, which resulted in major improvements to the regional wastewater treatment system.

As a result of financing disputes over the WPAP between the County and the City Commissions, the Wisconsin Legislature created 1982 the current Milwaukee Metropolitan Sewerage District which is governed by a single Commission.

After extensive planning and public input, construction on the WPAP began around 1979 to repair and expand the entire metropolitan area wastewater conveyance and treatment system. The cornerstone of the WPAP - Milwaukee's Deep Tunnel System – involved 19.4 miles of Deep Tunnels dug 300 feet underground to help reduce sewer overflows and basement backups. It started operating in 1993.

Most of the Deep Tunnels are carved out in bedrock 300 feet underground by what is called a boring machine. MMSD built its massive storage system in three phases starting in the 1980s.

milwaukee deep tunnel boring machine works on the deep tunnel

The region invested $3 billion in the WPAP improvements with $1 billion going toward the Deep Tunnels. Water pollution from sewer overflows dropped drastically from the 8 billion to 9 billion gallons that occurred, on average, every year before the tunnels.

MMSD invested another $1 billion in an Overflow Reduction Plan that was completed in 2010. Part of the effort led to two additions to the Deep Tunnel system that currently stores 521 million gallons and stretches 28.5 miles long.