"There's a whole world down here that all the people above don't know anything about," said tour guide Margaret Hicks. "It's really a whole city beneath the city."
It stretches nearly 40 city blocks, about five miles centered around the Loop. For those who don't use it every day, yes, you need a tour guide. Hicks does it for a living, touring Chicago underground.
The Thompson Center is one of the many government buildings connected by the underground walkway system called the pedway. Head down the escalator, through the food court and you'll spot the official sign.
Part of what makes the system confusing is there isn't always a sign. And there is no official map.
"You have to have a really good explorer spirit to you, got to just head down there, give yourself an hour and just be brave," Hicks said.
Every section of the pedway has a unique personality. There was no master plan, which adds to the mystique. But besides getting you from Point A to Point B, you can find quite a bit underground. There's marriage court.
"We just filed our marriage license here for the first time," said one woman.
Starbucks, Dairy Queen and a super-secret express DMV.
You can walk from the Thompson Center, past City Hall, the Daley Center, to the Cook County Administration Building without ever having to step outside.
It all began between State and Dearborn.
"This is the original section of the pedway that started in 1951 between the Red Line and Blue Line, so there's a nice covered connection between the two," Hicks said.
Today, 59 years later, it's estimated more than 7 million people use it every year.
"It's easy access; it's nice when the weather is bad, don't have to walk outside," said one pedestrian.
The newest section opened last year, just steps from the original pedway under Block 37 where you'll find an underground mall with unique shops, like one dedicated to cream puffs.
From Block 37, you can pass under Macy's and head right into another gem of the city, the Cultural Center.
"If you have an adventurous spirit, come down. Once you figure it out, you'll be really glad you have that knowledge," Hicks said.