I Think this has been posted but anyway heres a pretty easy way under SW. First of all it's hardest with Gnomes and Dwarves cause there real small. Well go to the left entrance/exit of the Trade District when entering SW. Then when looking at the entrance/exit, go to the left side and start jumping almost straight at it, not inbetween wall and torch. Once you start floating then jump again and keep doing that till you get on torch, then jump on wall and start running left and you'll fall through the wall and your under Stormwind.
P.S. Don't fall through endless holes cause you will fall and die, but if you do, Spirit Rez, I repeat, SPIRIT REZ! Lol ok.
P.S.S. To get out, fall through a blue hole, portal, wall,etc. I haven't played WoW in awile so I kinda forgot.
P.S.S.S. You can go really far under SW.
There's some more useless stuff that I can add, but I won't bother cause nobody would care.