Screenshot of The Underbelly in-game map
The Underbelly is the name of the Dalaran sewer system which has its own separate map. Many races, but mostly goblins, dwell in these sewers and they could be viewed as the slum of Dalaran. The central area of the sewers is the Circle of Wills, a large hall in which the arena vendors, some dueling circles, and a small, currently unused cluster of buildings stand. Southwest of here is a section called Cantrips & Crows which contains the inn, the tavern, and the bottom exit pipe of Dalaran. The northeastern corner goes unnamed, and has a few vendors but not much else notable. Finally, further northeast, there is a small chamber in which the rogues of Dalaran operate the Black Market where reagents and rogue supplies can be found, as well as the vicious sewer shark Segacedi.
Dalaran Arena, one of the two Wrath of the Lich King arenas, are set here. The other is Orgrimmar Arena in Orgrimmar.
Gallery
North room at end of tunnel.
Entering the Underbelly from northwest entrance.
The inn (near northwest entrance; notice bank and tunnel out)
The bank (near northwest entrance; notice guild bank access)
End of long tunnel out (entrance to right of bank).
Tunnel out end from outside.
Central chamber looking southeast from northwest end.
Arena organizers' building in central chamber.
Entering the Underbelly from southeast entrance.
- Dalaran Arena.jpg
The Dalaran Arena loading graphic.
Notes
The tunnel out that can be accessed to the right of the Underbelly bank does actually exit the underside of Dalaran, leading to a long fall to the ground in Crystalsong Forest. A Slow Fall-type effect is required to avoid death due to fall damage. Also, it is possible to fly into and out of the Underbelly from here.
In the southeast part of the Circle of Wills is a circle with a drain, through which arena teams are sometimes visible, made up of "Gold" and "Green" classes. For example, a "Gold Mage" and a "Gold Paladin", with a "Green Warrior" and a "Green Priest".
In the Underbelly, green potions randomly spawn, which includes 
[Underbelly Elixir].
A 
[Giant Sewer Rat] companion pet can be fished from the Underbelly, but its drop rate is very low.
Rarely, your character will turn into a sheep (similar to the 
[Polymorph] effect) and a buff "Manabonked" will be applied. The next time you check your mail, you will receive a package with a letter of explanation and a one-charge wand called 
[The Mischief Maker] which has the same effect when used on another player.
