This is Operation: Gnomeregan that was implemented 7 Sep 2010. You must help High Tinker Mekkatorque to free Gnomeregan from Mekgineer Thermaplugg. This is for Alliance players only. The first quests can be completed by any level (I believe) but the actual battle outside Gnomregan can only be done by level 75-80. This event is in preparation for Cataclysm when the gnome starting area will be just outside Gnomeregan.
There is a similar event on the horde side for the Trolls, unfortunately I don't have any high level horde characters so i can't do a video of that event.
The third in the series of World of Warcraft Instance Runs. A quick solo run of Gnomeregan. I got lost at one point and that's why there's a cut in the middle, otherwise it's all done in real time.
Gnomeregan is located on the western border of Dun Morogh, not far from Ironforge. It was the home of the Gnomes until it was nearly overrun by a Trogg invasion. In a last ditch effort to rid themselves of the troggs, High Tinker Mekkatorque detonated a radiation bomb, designed by Mekgineer Thermaplugg. Thermaplugg was greedy though and wanted to be High Tinker and through deceit and trickery convinced Mekkatorque to flood the entirety of Gnomeregan with radiation from the bomb thus killing the vast majority of the gnomish race. Many gnomes survived, turning into Leper Gnomes. Most of the troggs survived as well and to this day the fight for Gnomeregan continues between the troggs and the leper gnomes, led by Mekgineer Thermaplugg.
Gnomeregan is a level 28-35 dungeon and you must be at least level 15 to enter. To get the completion achievement you must defeat Mekgineer Thermaplugg, a level 28 elite leper gnome in a mechanical battle-mount.