I found a way to get under Northrend from loken's bargain in storm peaks. It requires blink but it is possible unstuck could work.
EDIT: I think there is a small hole on the very left side of the invisible wall.
Start here:
Fly to the end of the tunnel:
Fly onto the ledge on the left and blink here:
You will end up on the other side of an invisible wall:
Fly back out underneath the floor:
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