Didn't see a thread about this one, and frankly I'm surprised I didn't think to bother with it until now. I was leveling up my new mage/engineer explorer toon, and did Alysra's quick quest into the Hyjal Barrow Dens to get Fandral Staghelm. The Barrow Den looks roughly similar to other Barrow Dens and I hadn't thought much of it before, but it is actually a totally unique model in there, and there are some big off-the-map spots behind invisible walls.
The only way I was able to get in there was through flying in from underneath the world. There are bat critters that fly beyond the invisible walls, and you might be able to shadowstep or rocket helmet to them or something, but those would both be impratical methods, because the water below will insta-crash you. You could lag/disconnect your internet once in that way and do a bit of exploring though.
Like I said, stay out of the water, and though there are some islands in the water that you can safely stand on, and a few of the structures along the walls are solid and you can stand on them, most of the walls and bridges and structures are insubstantial and you'll go right through them. Also, if you land on any of the solid structures, you will auto-dismount and not be able to mount back up, so choose your course wisely.
There are two areas in there to check out. A big main room with some solid structures, and a long, narrow chasm with no solid structures, but some nice light and fog effects.
As an added bonus, I found these two elf ruins on the way underneath the world in Talrendis Point! They may be old news, but they were new to me.