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This cave can be seen in the Northeast corner of Mulgore, near the top of the mountain, while taking the flight path between Orgrimmar and Thunder Bluff. This may be something that Blizzard started and never finished, or it could simply be flight path scenery similar to the Dun Morogh airfield. This area consists of a Tauren hut and a cave, both of which are completely empty.
The only way up there was very hard to climb, and if you were not specifically lucky, it could take more than an hour to jump the thin ledge. However after Patch 3.2, it's possible to take the The Zephyr between the two cities, and jump off onto the top of that mountain.