Stormwind Cellar

The Stormwind Cellar, or "underneath Stormwind", is an area reachable in at least one way. The most reliable one I'm aware of is to use the Dun Morogh Highway, then slow fall (or ride) down stepped plateaus into a hidden part of the Elwynn Forest zone.

You can clip through the walls around the harbor to step out into Stormwind proper. This seems like an interesting attack vector for the Horde.