Start by getting under Stormwind gate:
http://video.google.com/videoplay?do...+SW+gate&hl=en
Thanks Dragos
Not bashing you Dragos, but you don't need unlimited breathe in that water.
Ok, now head under the road thing to the south west part of the gate. Head up the hill as high as you can then jump. If you did it correctly, you will have jumped ontop of the gate.
Happy exploiting
P.S. I saw only one thread about this, but the video on it expired, so I put up a new thread.