There is a ladder you can climb in fuel plant in the upper part of the map that leads to catwalks outside.
Climb to the top, then jump on the railing beside you.
Walk on the railing all the way down to a dead end, then back up and sprint jump towards the wall but slightly to the left.
You will go straight through, and where you go from there is up to you, but don't fall off the map unless you want to start falling forever.