When you are in the train station map in multiplayer, go to the left side of the train track. Go a little bit forward and you should see a section of wall shaped like \____/.
Take cover on it and do a Roadie Run across it. If you do it right, you should be crouching on top of the low wall (keep trying if it doesn't work the first time). Jiggle the left stick until you stand up.
Follow the wall and you can walk on it. Now for the fun part! Keep going until you see a small tower that is next to the edge of the rubble. There should be a little slant on the side.
Run at it and you will fall down onto the outside of the map. You can not be killed (that I know of-I don't swear by it). Run around and you can go through the mountains. It's hilarious!
If you throw a grenade down the blackness it will keep going for a long time and you can see it. (Send me a friend invite if you can't do it. I'll be happy to help.)-----Alienspider1