While on the quest Bringing down the Iron Thane, featured here:
Bringing Down the Iron Thane - Quest - World of Warcraft
As you're inside the golem, if you click the X on your bars, to dismiss the golem, while you're on one of the elevators in Dun Argol by the Iron Thane required for the quest, you will fall through the ground and land in an elevator shaft.
Picture below:
Now, while you're in the elevator shaft, you can mount. Personally, I was far too poor to afford a flying mount at 77, yeah, I know, I'm worthless. But I did walk through the wall on my raptor, and you will fall into nothingness and die.
However, with a flying mount, I am positive that you will be under northrend like the many similar glitches. I will reply when I have my flier and follow up with more screenshots.
In the meantime, those of you with fliers, enjoy!