If all you've ever done is aboveground exploring (abandoned houses, cool graffiti, secret quarries), it's time to go down under... and by that, I mean underneath Brookfield. This tunnel is close to home, free of human interference, and probably infinite (I've never been to the end). I'd recommend it for anyone who isn't claustrophobic.
The GPS coordinates of the tunnel entrance site are 43.122871, -88.114238. Park in the Kohl's Corporate Headquarters parking lot and approach the entrance shown. Follow the first tunnel and make sure to take the first ladder that you see to reach the second tunnel. From there, you can go whichever way you want - just remember how to get back to the entrance! Worst comes to worst, turn off your flashlights and follow the daylight or moonlight out.
TIP: In the image to the left (or above if you are on mobile view), the Kohl's center is tagged with a red pin and the tunnel is located on the left side of the pond (it is that gray area by the sand).
Have you been to these tunnels? What did you think? Leave any tips of your own or observations in the comments to help other explorers!