Here is a cool trick I found while Exploring in SW. You can get under/inside the stockades building in Stormwind and then use the summoning stone to summon others to you. You can then fly down into the hole under the stockades building and fly around under the terrain.
While I'm at it here are some addition tips for flying under the world:
- If you have the headless horseman's mount or spectral steed use them because they are water proof.
- You can fly all the way down to the kill barrier and land on it, don't move or you will fall and die but you can remount, cast, eat etc while standing still on it.
- Anywhere there is a cave or some pathway that passes through terrain there is a hole in the terrain to allow characters to pass through, its just a matter of getting through the cave or structure walls to reach the hole.
- Nether Rays are the tallest mounts that I know of and they can fly making them excellent for mounting through walls.