There's many posts yes, but they are all shown with an Underbelly Elixir - Item - World of Warcraft in wasp form. This will show you how to get into Dalaran Arena and other spots with a flying mount.
1. Mount up with a windrider or gryphon. A drake is too big for this.
2. Fly directly down into the well near the North Bank. If you hit any sides, chances are you won't make it to the right place.
3. Map of steps 1. and 2.
4. Make sure you're on the side with the stairs out and pipes. (shown on the minimap)
This is also the tricky part, fly to the ceiling and aim down for the bigger pipe.
5. This is where you should land.
6. Move forward above the water pool part. then directly down. (pink spot is shown, where you came from)
7. Move to the top left corner. Fly to the area shown.
8. This is where you should land.
Why is this useful, besides exploration?
AFK fishing with bot. [Sewer rat can be caught here]
Arena fights often occur with NPC's
Another area that you cannot normally get to
1. Instead of the top left corner, go to the top right.
rare npc that patrolls there
into a blocked area
enjoy!