|Underdark, not Underdog|
The Underdark is pretty much the underground civilizations and connections of your D&D campaign. It first appeared in Doug Niles' Dungeoneers Survival Guide as a general term for this region, and has been adopted by the Realms, but really it goes all the way back to Descent Into the Depths of the Earth, Module D1, which consisted of a string of underground encounters that sketches out the idea of a larger economy and interrelationship of various species. Just because it is not human-dominated doesn't make it any less important.
|D1 - Classic Underdark|
The Underdark as a setting is pretty uniform, whether it is in
Oerth, the Realms, or the Rock of Bral. Natural features. Rough caverns. Stalactites. This is where the drow and the mind flayer are comfortable, where herds of rothe (miniature musk ox) roam, and fungoid creatures like myconids and various noxious molds hang out. In many ways it is as typical fantasy as any dragon-haunted mountain or orc-populated plain. Its greatest challenge for players is safe locations where one can recover and heal up, as most of the inhabitants are hostile to humanity with only a few that skew towards the neutral-greedy end of the spectrum.
As a result, the Underdark, a dungeon world without all that finished stonework and doors, remains a tool in the GM's kit, capable of being adapted to any campaign at need. And it rates as a campaign of its own.