Note, I couldnt fit through this hole with my own character as it is an orc, however a smaller race such as an undead should be able to fit through it..
Step 1) Head to the west tunnel between Shattrath and Nagrand (78,55)
Step 2) Go up the right side of the tunnel wall, up the wall.
Step 3) Jump through the small hole in the wall, and you should be underneath the tunnel, and able to fly around under Outland.
If someone with a smaller character could comfirm if this works or not, it would be appreciated.
If someone with a smaller race could test this, it would be great.