"Around the Civil War time, the first tunnels for pedestrian traffic were built," said John Russick, senior curator, Chicago History Museum.
Those tunnels were built under Washington and LaSalle in order to get people from downtown to the West and North sides of the city. The tunnels also served as an escape route during the Great Chicago Fire. Near the turn of the last century, a more extensive tunnel system was built to install telephone lines.
"And they built those tunnels large enough to have a small gauge railway system there," said Russick.
The freight system was used to deliver goods and coal to downtown buildings, as well as discreetly haul away trash and ash from the boiler rooms. At its peak, there were 60 miles of tunnels mirroring downtown streets.
With the advent of paved roads and cars, tunnels became obsolete and many were closed. The remaining tunnels were used for utility lines. In 1992, the breach in the Kinzie tunnel led to Chicago's underground flood. Many of the tunnels have since been sealed. Only city engineers and contractors are allowed in.
"The tunnels are six feet wide by 7 1/2 feet tall in a horseshoe shape," said J.J. Madia, Chicago Department of Transportation civil engineer. "You have to have training for confined space because you're in a limited area ? You have to walk the tunnel by flashlight; there's no lights."
In the sub-basement of City Hall there's a super-secret hatch leading to the tunnel system.
You'll have better luck traveling beneath another century-old form of transportation -- the Lake Street Bridge -- one of 36 moveable bridges in Chicago.
"The ones downtown operate about 150 times a year, mainly for pleasure craft. The ones in the Calumet area ? operate 5 to 6,000 times a year because they operate year-round," said Vasile Jurca, CDOT civil engineer.
The control room of the bridge house is the brain of the operation. Underground is the brawn.
"This is known as a bascule bridge. Bascule is French for 'seesaw' or 'teeter-totter,'" said Jurca. "Although it's a huge piece of machinery, it doesn't take much power to move it since it's so evenly balanced. All it takes is two 60 horsepower engines, which account to smaller than a mid-size sedan."
The first step: Warning bells ring as the gates come down. Crews make sure the bridge is clear, and the operator disengages the locks and applies power. As the bridge begins to rise, the counterweight 40 feet below does the rest. Three crews of bridge tenders leapfrog to every third bridge to keep the operation running smoothly. Once all the boats are clear, they do the whole thing in reverse. The whole process takes about 12 minutes.
One of Chicago's most famous underground routes is also the one most available to the public -- Lower Wacker Drive. It's famous because it's been featured in many popular films, such as "The Blues Brothers" and "Dark Knight."
"Wacker Drive is one of the few streets that goes north-south and east-west, and it connects Lake Shore Drive to the Eisenhower, 290 Expressway, so it's a great shortcut to get through the central business district with very few lights," said Michelle Woods, CDOT assistant project director.
Wacker Drive was part of Daniel Burnham's plan of Chicago in 1909; the idea was to separate pedestrians and vehicular traffic on the upper level and put all industrial and business traffic on the lower level.
Right now, the north-south section of Lower Wacker is closed in preparation for a total reconstruction of Upper and Lower Wacker from Randolph to Congress. In two years, Chicago's favorite underground shortcut will be back in business.
While Lower Wacker never connected to the freight tunnels, the east-west portion, which opened in 1923, is considered historic. The north-south section, which opened in the 1950s, is not.