Found a new (to me) way to get under Northrend and did a little poking around. Found some neat stuff! Apologies if any of these have been posted before.
This is where I got under. In Zul'drak, using a mushroom chair and vrykul horn and then mounting up, there's a hole through the ground inside the wall nearby.
First thing was the fleet of green pizza slices beneath Mam'toth. They correspond to particle effects on the surface.
Then was a long, thin, purple cone underneath Sseratus. You could fly into it, but not back out of it, although you could then follow it up to the surface and back out into the world.
Lots of stuff partially underneath the surface below Ulduar, but this one large pill-shaped thing was the only free-floating, totally-disconnected-to-anything, completely under the surface item I found.
Beneath Dun Niffelem there were these four floating diamond-shaped objects. Maybe they corresponded to quest-giver objects that phased in as you gain rep with Sons of Hodir. I dunno.
Then I headed toward a specific area I wanted to check out after questing there the other day and having my curiosity piqued: Gjalerbron in Howling Fjord.
There are four furnace/chimney things with crypts in them. You can fly in and out of them with no problem.
But what I was after were these rooms you can see below four circular grates in Gjalerbron. There are NPCs involved with a quest in these rooms, but you complete the quest in an underground area that didn't appear to connect to these four circular rooms.
I flew into one and was auto-dismounted. I was surprised to find that the NPCs didn't aggro on me, and I could not attack them either. There is no way in to these rooms aside from flying in under the world.
What was also surprising was the amount of levels this hidden area had. There were three or four tiers at the top with NPCs in them, walls, and detailed decorations, then three or four tiers below that with spider-webs obscuring the view of the central circular pit but none of the other stuff, then 10 or so more tiers below that which were totally empty, just floors, but added to the look of depth from the surface. There was a last platform in the middle at the very bottom.
All of the tiers were solid, and all of them would dismount you. I was able to go back and check the different levels by speed-jumping and slowfalling into the water below the lake to the south-east of Gjalerbron, then flying back.