Is there a similar situation for alliance?

It's not a Horde only area. It's accessible by use of the Wormhole Generator: Northrend, an engineer only device.

Seriously? Damn, I'm going to be logging on my engineer three times a day until I see this place.

I had no idea these secret rooms existed. On one had, I'm sad that I'll never see them, but I'm also glad that Blizz puts these little touches in the game. Maybe someone could link some screenshots of these places for the rest of us?

THATS WHAT THAT MEANT!!!

I thought I was insane. I am the only max level egineer in my guild, so they were not help. Everytime I use my wormhole I wonder if I could get Underground again, since I didn't use it the first time I got it (going to TOC for a raid)

Oh! I did not know that about the Wormhole Generator and here it sits in my bank....... TYTY

I didn't know there was an Auction House in Dalaran until a few weeks ago when my hubby who's an Engineer.. told me he was stopping by the AH and I /gasped! upon the sight of a little auctioneer only usable by those of the Engineering profession!!! right in the middle of Dal!

This room is my favorite. The dragonling schematics net me over 1,000 gold per visit.

Technically...

"There is the one in the Violet Citidel that "Higher Learning" gains you access to and there is another underneath the Sunreaver Sanctuary called the Dalaran Underground."

This isn't quite true. You can simply fly up to one of the towers above the Violet Citadel and you'll find a balcony with door right into the room where the Archmage is at. The trinket from Higher Learning simply affords a teleportation method to reach this room, but you can also just fly up there.

No, actually, that room with the Balcony is the Purple Parlor. You can teleport up there from from a portal in the Violet Citadel. The room from Higher Learning has no doors or windows.

You can also access the Dalaran Underground with some creative maneuvers while flying in the wasp form effect from the Underbelly Elixir. You can fly over the invisible barriers covering the "wells" beside the paths. Skimming the surface of the water in some of them allows you to fly inside Dalaran's floor, allowing you to access the Dalaran Underground. Just be careful not to fly up through the floor; the floor and walls are one-way only.

There are also two other "secret rooms" you can access this way. You can get into the Dalaran arena that is visible through the grates of the Circle of Wills (arena section) in the Underbelly. Occasionally two-man NPC arena teams will enter the arena and fight.

The other area is under the hallway between the Circle of Wills and the Black Market. It's inhabited by the rare but unkillable Vern, the crocolisks who can occasionally be seen through the grate above, patrolling around below.

I just learned about the engineer room myself! I was planning on trying to port closer to Grizzly Hills and there was an extra option "underground" so I took it and ended up in the room there.