I searched a bit and didn't see this, I hope it hasn't been posted. If it has, it must be really old, my apologies.
A simple straight wall jump set to get on top, then just hop off the edge to get under. Beneath the instance we found a single hole through the world, not much else. Took these Friday, Feb 8 on live servers.
Enjoy!
--Greengoo
Starting Point
http://img151.imageshack.us/img151/2453/smstartfs3.jpg
Jumping (1)
http://img151.imageshack.us/img151/415/sm2wq0.jpg
Jumping (2)
http://img406.imageshack.us/img406/1971/sm3ms5.jpg
On the roof
http://img406.imageshack.us/img406/4674/smroofyl7.jpg
"Basement" Access (slowfall/bubble/etc)
http://img151.imageshack.us/img151/5312/smdropbk9.jpg
Don't fall in!
http://img212.imageshack.us/img212/6164/smholekz3.jpg