Note: You need Old School mode enabled to do this.
Go to the very east part of the map. There are 5 crates blocking the way to the outside of the map. Turn around and you will see an old fridge and a dumpster, jump on the fridge then onto the dumpster. There will be a wall directly to your right and a shorter wall in front of you a little bit away. If you sprint-jump from on top of the dumpster you will be able to jump over the wall, and make sure you are on the top of the slant of the lid of the dumpster, this makes it a lot easier. This may take a few tries. From there face west, and go straight, you will see an allie with no floor, simply walk over the hole, and viola, you're under the map. You can't go under buildings though, but it's still fun.