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.
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 or immunity 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".
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.