The entrance for the Gnomeregan Instance is located inside the starting area for gnomes. The story of this dungeon is the ongoing attempt to reclaim the Gnomish capital after it was bombed from within to hold off some attacking troggs. It is also one of the most hated instances in the game.
You enter the underground city via a shaft drilled into the mountainside. Once underground, you are surrounding by a puke green aesthetic (it's all of the radiation) and Gnomish architecture, which is pretty much "put more gears on it."
Multiple levels make it hard to navigate. Tight corridors make it so that one false step can get you swarmed by a ton of enemies. Every time you kill an enemy in here it makes an annoying "waaaaaaahhhhhooooa" sound effect. It also takes forever. Anyway, the final boss is
Dr. Robotnik Mekgineer Thermaplugg, the guy who originally had the idea to nuke the city from the inside. It's unknown whether he did this on purpose.