If you are an engineer, you can have a funny experience by going “Underground”. There, there is a vendor who sells three very rare engineering recipes (not bound on pickup).
First thing first: You will have to build the Wormhole Generator Northrend. If you leveled up at the Lich King time, there is a big possibility that you have it because, back then, it was necessary to make it to level engineering up. It is not necessary anymore today, so many players choose not to build it. Good thing, because then, there is less competition.
To build the Wormhole Generator Northrend, you will need a engineering level of 415 and 8 x titanium bars, 2 x eternal shadow, 2 x eternal water, 2 x eternal fire and 2 x eternal air.
Once the wormhole is built, you can just right-click it, and it will activate. It has a 4 hours cool down.
At the opposite of the wormhole Pandaria, which sends you a random place on Pandaria, this wormhole let you choose, where on Northrend, you want to go. You can’t go everywhere and you have the choice between Borean Tundra, Howling Fjord, Sholazar Basin, Icecrown and Storm Peaks. Always.
However, sometimes, you will have a 6th option in the bottom: Underground…
If you take this option, you will be sent to an oubliette in Dalaran, which is, in fact, in front of the Horde’s part of town (but you don’t risk anything). In this oubliette, you are alone with the undead vendor, Kaye Toogie.
This vendor sells three rare engineering recipes. The recipes are:
Schematic: Arcanite Dragonling
Schematic: Mithril Mechanical Dragonling
Schematic: Mechanical Dragonling
Note that only the Arcanite Dragonling is exclusively sold by Kaye Toogie. The Mithril Mechanical Dragonling is also sold by Ruppo Zipcoil, in the Hinterlands and the Mechanical Dragonling is sold by Gnaz Blunderflame in Northern Stranglethorn as well (both also limited supply).
The vendor might have nothing for sale when you visit her. It seems, that there is a respawn time of 24 hours on these recipes. So you might need a bit of patience. Any engineer from your server (or one of the servers on your cross-realms) can buy them before you.
You can use the recipe to make the items yourself, of course, or you can try to sell the recipes. Depending on recipe and on server, they go from 200 gold to 700 gold a piece.
And YES, I had my little, scared, gnome mage in this dark, cold, moist oubliette, together with an undead and she wasn’t authorized to come out before she had all three recipes. She was down there three days. You have to teleport or HS out of there. There is no way out.