Where to go - Ymirheim, Icecrown
What to do - Get in the jumpbot (in flight form) and head just above the horde PvP questgiver.
Use it's number 3 (jump boots) ability as shown:
http://img11.imageshack.us/img11/151...3009182646.jpg
Get out of the vehicle after it jumps through the rock face, but before it comes out the other side.
Presto! You're under the world!