What's Under a Twin Cities Street

There is a creek under downtown Minneapolis. Bassett Creek was an open stream when the city grew up around it, an industrial dump by 1867, and locals were calling it a "mammoth sewer" by 1876. In 1892 the city straightened it and put it in a tunnel. It still flows today, under the warehouses and the entrance ramps, and its last 1.7 miles to the Mississippi are entirely underground. Wikipedia

A buried creek is the extreme case of a general rule: anything a city needs that is ugly, dangerous, or freezable ends up under the street. In the Twin Cities that means five separate systems, run by five separate kinds of organization, stacked in loose layers: communications near the surface, then gas, then electric, then the storm drains, and at the very bottom, in tunnels bored through soft sandstone, the sanitary sewers. This post walks down through them in order.

You can see the whole stack without digging. Before any excavation, Minnesota law requires a locate request to Gopher State One Call, and utilities then get 48 hours to send locators out to mark their lines on the pavement. GSOC The spray paint follows one national color code, so a torn-up street reads like a legend: orange for communications, yellow for gas, red for electric, green for sewer, blue for water. The colors of this post are the colors of the paint.

Communications lines are the shallowest thing in the ground, usually within the first couple of feet. They can afford to be: a fiber line doesn't freeze, doesn't leak, and doesn't explode, so it doesn't earn the expensive deep real estate. What it gets instead is conduit, plastic duct a crew can blow new cable through years later without digging, and the small square handholes you walk past in every boulevard.

The physical internet of the Twin Cities converges in ordinary-looking buildings where networks hand traffic to each other. The biggest is the 511 Building, an old warehouse at 511 11th Avenue South in Minneapolis that works as the metro's carrier hotel: the regional exchange, MICE (the Midwest Internet Cooperative Exchange), peers 158 networks there, in the Cologix MIN1 facility. micemn.net PeeringDB Traffic between two Minnesota customers on two different providers can turn around locally instead of detouring through Chicago, which is the entire point of having an exchange in town. It is on the map, one block from the stadium.

The gas main running past your house is most likely yellow plastic, buried a few feet down, and owned by CenterPoint Energy, the company Minnesotans knew for decades as Minnegasco. Wikipedia Natural gas is odorless, so the smell you know is added at the plant: a trace of mercaptan, the rotten-egg sulfur compound, so a leak announces itself to anyone with a nose.

Minnesota's real enemy underground is not the gas. It is the frost. The ground here freezes deep enough that the state building code sets minimum footing depths by county: 3 feet 6 inches in the southern zone, which includes the metro, and a full 5 feet in the northern counties. MN Rules 1303.1600 Anything that can burst or heave has to live below that line. The frost is also why the street above every utility slowly fails: water gets into cracks, freezes, and jacks the pavement apart, one winter at a time. Potholes are the surface's opinion of the climate.

Electricity is the one utility you can watch enter the ground. In residential Minneapolis and St. Paul it mostly hangs overhead, but at the edge of each downtown the wires dive: into concrete duct banks under the sidewalks, splice vaults under those steel doors you walk over, and transformers humming in basements. Burying the core is a capacity decision: downtown load is too dense for poles, and a network fed from many directions at once can lose a cable without losing a block.

The wires lead back to Xcel Energy, the Minneapolis-headquartered utility serving about 3.9 million electric customers across eight states, and in Minnesota that power leans hard on two nuclear plants sitting on the same river the sewers drain to, Monticello upstream of the cities and Prairie Island downstream near Red Wing. Wikipedia

The reason the grid was born here at all is the falls. St. Anthony Falls is the only major waterfall on the entire Mississippi, and by 1882 its water was spinning a Brush central power station, the third central station in the country, the same year Edison's Pearl Street plant opened in Manhattan burning coal. Wikipedia The falls have never stopped working: the Hennepin Island hydro plant, built in 1908, still generates for Xcel today. Wikipedia

Every street grate in the Twin Cities is the mouth of a second, parallel drainage system whose only job is moving rain and snowmelt off the pavement before it floods a basement. It ends at the lakes and the Mississippi, with no treatment plant in between. Whatever washes off the street goes with it.

That pathway is how the metro acquired its quietest pollution problem. Road salt dissolves, runs down the grates, and stays: one teaspoon of salt is enough to permanently pollute five gallons of water, and once chloride is in a lake there is no feasible way to get it back out. MPCA Every bag of ice-melt on every driveway is a permanent donation to the chain of lakes.

Bassett Creek, from layer zero, is the storm system's founding member: a real creek doing this job since before the city could build pipes, entombed in 1892 and still on duty. It even got an upgrade this decade, a new stretch of tunnel built between 2017 and 2020. Wikipedia The biggest tunnels under either downtown are storm tunnels, many tall enough to walk through. Brick Their whole job is getting a July cloudburst to the river before it finds your basement.

Under the glacial soil, the Twin Cities sit on a sandwich: a hard cap of Platteville limestone over the St. Peter Sandstone, a bed of soft, nearly pure quartz sand that barely holds itself together. Wikipedia Soft rock under a strong natural roof is a tunneler's dream: crews carved sewer and utility tunnels through it by hand, a labyrinth documented at book length by the geologist Greg Brick, whose survey of it also describes Schieks Cave, a natural cave sitting under downtown Minneapolis that city workers reach through the sewers. Brick, Subterranean Twin Cities book source

Everything you flush drops through city pipes into that deep layer, where regional interceptor tunnels collect both cities and carry the flow along the river to Pig's Eye Lake in St. Paul. The Metropolitan Council runs the regional system, nine treatment plants in all. Met Council The Metro Plant there treats about 225 million gallons a day, roughly half the wastewater generated in the entire state, for a service population over two million. MDH

The plant is the good ending of an ugly story. For the cities' first 80 years, everything went to the river raw. By the 1920s an estimated three million cubic yards of sewage sludge lay settled on the river bottom above the Ford Dam, with mats of it floating downstream. When the Pig's Eye plant opened in 1938, the Twin Cities became the first major metropolitan area on the Mississippi to treat its sewage at all, and the plant drew engineers from around the country to see how it was done. MDH

One more fix took another half century. The old city sewers were combined, one pipe for both sewage and stormwater, so every hard rain flushed diluted sewage straight to the river through overflow outfalls. Minneapolis started separating the two systems in the 1960s, accelerated hard in 1986, and by 1996 had eliminated all but 8 of its 34 overflow regulators; the leftover corners were separated with later street projects, and by 2018 the city moved to retire its overflow discharge permit entirely. City of Minneapolis council record

That soft sandstone nearly killed the city once. St. Anthony Falls exists because the limestone cap resists the river; every tunnel and millrace cut into the rock weakened it, and between 1857 and 1868 the falls were eroding upstream at 26 feet per year. Wikipedia If the cap failed, the falls would unravel into rapids, and the waterpower that ran the flour mills, which is to say the reason Minneapolis existed, would be gone.

On October 5, 1869, it almost happened. A tailrace tunnel being driven under the riverbed, the Eastman tunnel, got too close to the bottom of the limestone, and the Mississippi broke through into it, scouring out the tunnel and caving in part of Hennepin Island. Temporary dams held. The permanent fix took the Army Corps of Engineers seven years: a concrete dike, finished in 1876, driven down as much as 40 feet below the limestone and stretching 1,850 feet across the entire river channel, sealing off the tunnel and any future path under the falls. Wikipedia

Then the Corps went further and replaced the waterfall itself. By 1880 the crumbling natural edge was covered with a sloping timber apron, rebuilt in concrete in the 1950s, and that apron is what you see from the Stone Arch Bridge today. The only major waterfall on the Mississippi has been an artificial structure since 1880, standing on a buried concrete wall almost nobody in the city knows is there. Wikipedia Wikipedia

Which is the whole underground in one object. Nothing down there is glamorous. It is a wall, some pipes, a creek in a tube, a cave full of sewer access ladders, and a plant processing half a state's flushes. All of it was built because the surface city stops working the moment any of it doesn't, and nearly all of it works so well that you can live here a whole life without once thinking about what the street is a lid over.