workers inspecting the 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 Tunnels?

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.

MMSD's Deep Tunnel

MMSD's Deep Tunnel

Lake Michigan lakeside

The Deep Tunnel has kept more than 125 billion gallons of pollution out of Lake Michigan.

combined sewer system

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.

Deep tunnel construction


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


Deep Tunnel Map

MMSD service area