Just to make it clear I found this by my self and DID NOT find anything when searching. So let's try not to make this a flamefest, aye?
This is a way to easily get the [Going Down?] achievement.
See that hole up on the wall there? That's where we're going to jump down.
Now how to get there.
This is what you see when exiting the ring containing the AH to the one furthest out thorough the west exit.
Run through that not-so-healthy water and over to the stairs. Have a light jog up the stairs and turn your nose into the tunnel, then walk forward till you hit the wall.
Continue till you run into a Undercity Guardian.
Continue right up ahead until you stumble upon a larger tunnel. Now look right and you'll be gazing over this .. uhm well .. ugly city.
Just walk off the edge and you'll have your achievement in no time.
Edit: I do know there's another way from the outside of UC, but I presume most people will be inside.
Edit II: Instead of telling me all the easier ways, why don't make your own thread with fancy pictures and flashy text?