Go to the Fort Staunton area and, after you do the mission where you blow up the tunnel, there will be several destroyed buildings farthest to the right of the bottom of Liberty Campus. Go the way closest to the river and continue straight until you are able to go to the right, where you can see a cracked broken wall that you can almost jump over. If have trouble jumping it, enable the "Better handling vehicles" code and press Down to jump over the wall while in a car. Jump over the wall to go through the rubble and fall, eventually reappearing on top of the rubble. You will then fall again. Use the "Suicide" code to exit. While falling, you can exit your vehicle if you used one.