Go to the Comm room door and open it. Then get a train or a sprinter after you.

Go to the left side of the hut and lay down putting your head against the wood post. Then let the train push you through or get the sprinter to down you.

At the instant you down, push forward on the left analog stick, which should push you under the hut.

Then get your friend to revive you and swim under the tree (don't go too deep or you go into a black hole).

Then go left to the metal plate, then go left and then go straight to the hill for a air bubble

After that keep going straight (slightly left) to another air bubble (should be under a tree), then go to the end of the map (do not go past the fake trees or you'll fall off the edge of the map).

Then look for the mud again (look up a bit for air) then when you find the mud look for a crease, then walk forward and you're out of the map!

Warning: zombies can still kill you.