deep tunnel in milwaukee

Milwaukee Deep Tunnel

The Deep Tunnel has prevented more than 125 billion gallons of pollution from getting into Lake Michigan. Thanks to the tunnel and many other improvements, MMSD has captured and cleaned 98.4% of all the stormwater and wastewater that's entered the regional sewer system since 1994. The goal nationally is to capture and clean 85% for the more than 700 cities with systems like ours.

Why Do We Need Deep Tunnel?

Water reclamation facilities can efficiently clean only a certain amount of water a day (about 630 million gallons for Jones Island and South Shore combined). 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.

Prior to the Deep Tunnel, the Greater Milwaukee region used to pollute Lake Michigan with an average of 50-60 overflows a year and now we average 2.4.

combined sewer system graphic

Does the Deep Tunnel Prevent Sewer Overflows?

Although the Deep Tunnel can hold 521 million gallons, our climate is changing, and increasingly intense storms and record-setting rainfall in the Greater Milwaukee region makes 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 2025 calls for investing $1.5 billion in clean water infrastructure, flood management, and debt financing to help protect public health and Lake Michigan.

Deep Tunnel FACTS

Phase One

Length: 19.4 miles

Diameter: 17ft. to 32 ft.

Depth Underground: 300 ft.

Storage: 405 million gallons

Phase Two

Length: 7.1 miles

Diameter: 20 feet

Depth Underground: 135 - 175 ft.

Storage: 89 million gallons

Phase Three

Length: 2 miles

Diameter: 21 ft.

Depth Underground: 300 ft.

Storage: 27 million gallons

Total storage of all three phases: 521 million gallons


MMSD service area

Why Do We Have a Deep Tunnel?

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. Both the Illinois lawsuit and DNR agreement led to the development of the Water Pollution Abatement Program (WPAP), which resulted in major improvements to the regional wastewater treatment system.

As the result of financing disputes over the WPAP between the County and the City Commissions, the Wisconsin Legislature created in 1982 the current Milwaukee Metropolitan Sewerage District that 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 Tunnel is 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

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

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. Our current financial forecast through 2025 calls for investing $1.5 billion in clean water infrastructure, flood management, and debt financing to help protect public health and Lake Michigan.