Searched for another post like this didn't come up. Sorry if it has been posted.
Me and my friend discovered this when we were dueling in places we weren't supposed to. (SW, IF ect.) These are the steps:
1. Go to Stormwind, and travel to the valley of heroes.(Front Entrance)
2. Find a warlock, at least level 10+ HAS TO HAVE FEAR. (Get someone preferably your level, you don't want to resist the spell constantly.)
3. Hop onto the ledge of the statue on the either first(coming from inside stormwind) or last (coming from elwynn in) on the right(elwynn entrnc.) left(inside sw)
4. Now have the warlock duel you, and have him cast 'Fear'.
Results :
Most of the time you will be glitched, fall down and into a place under Stormwind City. Sometimes you will fall right into the water, it is not perfect but I'd say 70% of the time this works.
Reason: It is just fun, also if you follow a certain path you can get behind that mysterious gate with an instance portal in SW.(I'll post screenshots if requested) You can also walk as far as The Searing gorge and other places. - Hearth to get out, watch out for bottomless pits