In the Rockford area, directly beside the hospital is a construction site. Get a PCJ 600 and ride it to the red bar at the construction site's entrance and break it. Then, back up and go fast as possible. Go up the dirt pile ramp inside the construction site. When you are on the first roof, jump over to the second roof while still on your bike. Then, go to the wall to your right and ride slowly into it. Note - You may need to do this more than once. If done correctly, you should fall into the wall. When you are done falling, you will be in the unfinished tunnels below. The only way to get exit is to die.