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"Walks Through Walls" shows you how to reach Lake Dumont in Feralas. This lake is surrounded by high mountains, and is only reached using a flying mount... or some clever wall glitching and slowfalling.
The NPCs on the island are friendly to both Alliance and horde (thanks, R4ndomizer). The lake area connects Feralas with Mulgore and Desolace, and you can travel to all three zones easily from here.
Check out the video below.
Enjoy!