Karri DeSelm works in the JW Marriott Building, on the corner of LaSalle and Adams in downtown Chicago. Her building, the last designed by famed architect Daniel Burnham, was completed in 1914, and underwent major renovations two years ago. Karri says that at the time, her boss told her that she had been down deep into the building’s basement, where she had seen the entrance to a secret tunnel that ran underneath the Loop.

That got Karri wondering:

“I have heard there is a network of layered tunnels under the city. Is this true, and if so, what was the purpose of the tunnels when they were designed and built?”

For starters, it’s true — there are many tunnels underneath the Loop. We found no fewer than six different sets of tunnels, including the tunnels connected to Karri’s building.

Each of the tunnels we found was at some point, or continues to be, a critical part of Chicago’s infrastructure. The city would be lost without these tunnels. Sometimes they’re hidden, and sometimes they’re just overlooked, taken for granted by the people who walk above them. But trust us — 2.8 million people would notice the tunnels’ absence because they’d have no reliable source of clean tap water, no flood control and no crosstown “L” service in the Loop.

And the tunnels that aren’t still in use are more than just odd architectural remnants or historical curiosities. They may be obscured from sight and from memory (or even sealed off), but they’re still an important part of the city’s built environment. As one source put it, we ignore the tunnels at our own peril. When we erect new buildings downtown, we do so in a densely layered maze of infrastructure, both old and new.

To help wrap our heads around Karri’s question, we worked with Erik N. Rodriguez of The Illustrated Press. Based on our reporting, he created the graphic above, which shows six different kinds of tunnels, how deep underground they are and how they’re situated relative to one another. Note, though, that the drawing is a composite; it shows what can be found at different depths across the Loop, but not necessarily beneath any single street address.

1. The Pedway

File Chicago’s Pedway under tunnels you may not know you know. You may have seen the system’s distinctive black and gold compass logo marking the entryways of skyscrapers downtown without knowing what they signified.

Short for “pedestrian walkway,” this maze-like system of semi-public hallways connects the basements of more than 50 Loop buildings, including municipal buildings like City Hall and the Thompson Center, shopping centers like Macy’s and Block 37, and a few newer residential buildings, like the hypermodern Aqua tower. The Pedway also snakes through two CTA stations, a Metra station and several underground parking garages along Michigan Avenue.

Although the Pedway provides a climate-controlled alternative to Chicago’s sidewalks, it’s more than just a thoroughfare. Under its fluorescent lights and beige ceiling tiles you can get your haircut, get a clock fixed, grab coffee, shop for a blender or order new license plates.

Perhaps that’s why Amanda Scotese offers walking tours of the Pedway through Chicago Detours, her unconventional tourism company. As Scotese’s carefully researched Pedway map illustrates, this system of tunnels is a disconnected mishmash. Although the Chicago Department of Transportation technically oversees the Pedway, many sections are owned by other government entities, while still others are privately owned and controlled by the management of whatever building they pass underneath.

Case in point: During a recent afternoon rush hour visit to the Pedway, Scotese, our question-asker Karri and I were stymied by a section of the Pedway under City Hall that closed promptly at 5 p.m.

2. CTA tunnels

File these tunnels under those you probably take for granted. Although the city prides itself on its extensive network of elevated trains, two downtown subway tunnels also move commuters through the Loop. These tunnels are now owned and operated by the CTA, and in 2012, the combined “L” stops inside the two tunnels served an average of 82,343 passengers every weekday.

The first tunnel runs beneath State Street and serves the Red Line. The second goes under Dearborn Street and Milwaukee Avenue and serves the Blue Line.

The city began digging the two subway tunnels in 1938, with the help of money from FDR’s New Deal and the Works Progress Administration.

Meant to accommodate crosstown “L” traffic, which could become snarled in the Loop, the tunnels range from 20 to 60 feet underground. Steel and concrete tubes 200 feet long housed the tunnels as they passed under the Chicago River.

As was the case with previous public works, the opening of the State Street subway tunnel in 1943 was cause for celebration: The curators of the transit history site Chicago L describe the festivities this way:

Between 10:25 and 10:45 a.m., ten special trains arrived at State and Madison to unload their passengers. At 10:47 a.m., Mayor Kelly cut a ceremonial red, white, and blue ribbon strung across the northbound track, officially giving the new subway to the city.

The Dearborn Street tunnel, delayed by World War II, was completed in 1951.

3. Freight tunnels

Of all the tunnels under the Loop, the 60 miles of freight tunnels 40 feet underground are the most extensive. They also happen to be unique to Chicago.

Dug by a private company between 1899 and 1906, these tunnels were meant to hide many miles of telephone cable. But transit historian Bruce Moffat says that somewhere during the construction process “the company’s promoters decided to build very large conduits — large enough for freight trains.”

Tiny freight trains, that is. The tunnels were only seven feet tall and horseshoe-shaped, with concrete walls and tracks running along the floor. Meaning … these freight cars were no bigger than small dumpsters.

This literal underground railroad delivered coal and freight to the sub-basements of prominent buildings in the Loop: City Hall, the Tribune Tower, the Merchandise Mart and dozens more.

The tunnels stretched from 16th Street to River North and the Field Museum. Remarkably, the tunnel system followed the street grid above so, to this day, you can navigate the freight tunnels using an ordinary Chicago street map.

That is if you could get inside. Most of the tunnel entrances were sealed in 1992, after a construction crew driving pilings into the Chicago River punctured the tunnels, flooding them and the buildings to which they were connected.

4. Cable car tunnels

Between 1882 and 1906 it was the cable car network, not the “L,” that served as Chicago’s main form of public transit. In fact, Chicago’s cable car system was once the largest and most profitable of its kind.

The technology that powered cable cars — a single, continuous underground cable — wasn’t compatible with the drawbridges that carried most other traffic over the Chicago River. Tunnels, though, could extend cable car service beyond the Loop to the city’s North and West Sides.

The first two cable car tunnels made West Side service possible via Washington Street and North Side service possible via LaSalle. These tunnels were expanded from remnants of pedestrian and wagon tunnels dug at the same locations in 1869 and 1871. In fact, just a few months after it opened, the LaSalle Street tunnel served as a major escape route during the Great Chicago Fire.

Sitting 60 feet below ground, these new cable car tunnels were deeper than their predecessors, but they also happened to be steeper. The new tunnels had a 12 percent grade — three times the rise of today’s CTA trains.

A private company built a third cable car tunnel between Van Buren and Jackson Streets in 1894. All three tunnels were later adapted for electric street cars, which replaced cable cars beginning in 1906.

But both means of transit ultimately fell out of use. When the “L” became ascendant the cable car tunnels were abandoned and sealed.

They’re still there, though, and there’s plenty more to read about their remnants.

5. Water tunnels

In 1867 Chicago built an intake crib two miles out in Lake Michigan to collect fresh drinking water for the growing city. Earlier efforts to collect water closer to shore had failed. If this fact inspires a big yawn from you, consider that at this point the city was still dumping sewage into the Chicago River, which fed directly into the lake.

This new crib fed water to the Pumping Station at Chicago and Michigan Avenues via a five foot tall, oval-shaped, brick-lined tunnel more than 10,000 feet long. At the time it was considered an engineering marvel. The crib-and-tunnel solution to water collection proved effective enough that Chicago built seven more intake cribs before 1935.

Those intake tunnels now feed through the city’s two filtration plants, but at least one tunnel was taken out of service and sealed when a portion of it collapsed near Lake Shore Drive in 1998. Officials also shut down portions of the drive during repairs, fearing the collapse might be a hazard for motorists.

But the city is tight-lipped about what other parts of this infrastructure remain in use. We wanted to know where the remaining tunnels are located and how deep underground they are, but the Department of Water Management denied our request.

Tom LaPorte, the department’s spokesperson and Assistant Commissioner, said the department feared such information might make the city’s water infrastructure more vulnerable to terrorist attacks.

“We’re the world’s largest water treatment facility,” he said. “Anything that’s going to put us at risk we’re not going to do, even for WBEZ.”

6. The Deep Tunnel

The Deep Tunnel is rarely referred to by its full name, the Tunnel and Reservoir Plan (TARP). But its nickname is apt; at a maximum depth of 350 feet it’s the deepest of the six sets of tunnels we’re treating here. When Chicago’s freight tunnels flooded in 1992, the water was drained into here.

This network of giant overflow sewers was built to prevent flooding and cut pollution in the region’s waterways. When heavy storms hit the Chicago area, excess rainwater funnels into the Deep Tunnel system rather than into the lake.

The tunnels’ depth is not the project’s only stunning statistic. As one writer put it, “the mega-project is one of the largest civil engineering projects ever undertaken in terms of scope, cost and timeframe.”

Phase 1 construction, a network of nearly 110 miles of tunnels designed to store 2.3 billion gallons of water, began in 1975 and was not completed until 2006. Three enormous reservoirs, designed to store an additional 14.8 billion gallons of water, are set to be completed by 2029.

The Deep Tunnel’s operator, The Metropolitan Water Reclamation District, kindly offered us a tour of the project’s south suburban pumping station, which, they told us, has a main chamber “the size of two NBA basketball courts.”

We declined their offer, but only for now. Curious City receives so many different questions about the Deep Tunnel and its economic and environmental impact that we’re planning a separate story for later this summer digging into that.

Tunnels aplenty, but running out of space

So Chicago is chock full of tunnels, at least downtown. There are other tunnels, too, in other parts of the city. Since I’ve started my reporting I’ve had sources regale me with tales of industrial tunnels that connect factories in Bridgeport, and listeners write in with tidbits about a tunnel that might run under Midway Airport.

But is the time for tunnels over in this city? Or could we see the construction of new tunnels in the future?

Sources we talked to said it’s unlikely. Most of the tunnels detailed above were built during Chicago’s greatest growth and expansion. Chicago had 330,000 residents in 1870, but it boasted over a million just 20 years later. Major works of infrastructure, whether financed publicly or privately, were needed to support and encourage such growth.

But now, Chicago’s population is declining — as many as 181,000 people left the city between 2000 and 2010 — even if some parts of town, like the Loop, have grown lately.

And between all the tunnels already under the Loop and other kinds of buried municipal and private infrastructure, it’s pretty crowded underground. While there’s no shortage of ongoing infrastructure projects abounding in Chicago, whether it’s the renovation of the Bloomingdale Trail (sorry, I mean the 606), upgrades to the Chicago riverfront or basic maintenance to the city’s sewers, only the Deep Tunnel remains on the city’s tunnel horizon.

That means that every tunnel down there now will one day be old. We may even abandon the newer ones someday in favor of better, more efficient solutions that haven’t yet been invented.

For our question-asker Karri, that’s a good reminder to pay attention to what’s there now.

“You work up in an office cubicle and don’t think about [what’s underground],” Karri said. Exploring that infrastructure now “can remind you of a flood, or the original purpose of the area, the history of it.”

“I guess that’s its value.”

Robin Amer is a producer on WBEZ’s digital team. Follow her on Twitter @rsamer.