I looked and didnt note that this was posted anywhere else so i decided id contribute my twocents to the exploration community, i am an obsessive explorer and find things on the odd occasion but heres my favorite:
Go to the southern Loch modan tunnel (dun morogh side) and jump up the hill to the right (going left across the top of the tunnel) until you get up there
Follow the wall south until you cant go any further then turn west and run in that direction until you get to the little plane that you see when you go over on the gryphon
Slowly jump down this hill so you dont kill yourself, you should get underneath the world with ease and be against a giant wall find a way down it is a VERY long fall if you dont get it right (i prefer the skyguard cape for this)
Now you are under the world and able to reach absolutely anything in the world that you wish to! If you run east then north you can get under ghostlands and into zul aman if your good at wall jumping, but i personally like annoying people by being under stormwind and telling them to find me and ill give them 100g you can watch the newbs running around like mad people but noone can find you
I will post a video if i get around to it later today