Whether there's a full-fledged Underdark under Kara-tur has not been stated, to the best of my knowledge. My feeling is there probably would be at least something, though since Kara-tur tends to be lacking large underground races/civilizations (dwarves, drow, duergar, purple worms, deep gnomes, et al, et al), it's probably much less developed.
I think Underdark says the Underdark extends at least 10 miles beneath the surface, and maybe 12 (afb atm), but at that depth you start getting gates to the demiplane of shadow and other weirdness. Whether it has an ultimate depth, I don't think has been explicitly stated.
If you have your heart set on multiple subraces, then I'd do it this way:
Several thousand years ago, civil war wracked [whatever city makes sense geographically for you, if we're talking Northdark, I'd say Eryndlyn, since it's large and has less written about it than Menzo or Ched Nasad]. After decades of struggle, one faction got the upper hand. Facing annhilation, the houses of the losing faction fled the city.
Due to their abominable habits [which could be just about anything, knowing drow], the victorious faction wasn't willing to settle for victory, and instead gave chase. For months, the losers fled deeper into the Underdark, fighting desperate rearguard actions against their more numerous enemies. Finally, deep in the Lowerdark, the chasers gave up. It had become too dangerous to continue persuit, and they were confident that what was left of the losing faction would soon perish.
The drow were only half right. What was left of the faction quickly disintegrated, with each family blaming the others for their defeat. Lacking an external threat to hold them together, the families scattered across their section of the Lowerdark. War, begun as simple raids for food and slaves, soon escalated into a multi-sided genocidal war.
Ever since, the surviving families have battled each other, oblivious to the outside world. Milennia of exposure to the hostile emanations of the Lowerdark have warped them, turning into creatures others would barely recognize as drow. To this day, they continue their war, slowly forgetting that any world exists outside of their battlegrounds.
But woe betide any force that gets between them!
That allows you to introduce as many odd variations as you want, as also explains why no one's ever seen them before: they've been too concerned with killing each other, and the only other group that knew they existed thinks they're dead. Then, you can either have your players get thrown into the middle (if they're powerful and *really* smart, they might even be able to do a Yojimbo, playing all ends against the middle). Or you could have one family send a young, expendable member up in search of new weapons to use against their enemies. This would, of course, cause the other families to follow suit, with the possibility of spilling their war across a larger swath of the Underdark.
DMD Yeah, after all these years I'm still trying to make the hollow world thing palatable, but it's a hard sell to my group.
I think it's easier to rely on something less obvious to the outside world. Either a hunt for magic/weapons by scouts, or an exodus caused by one faction finally getting the other hand and starting to wipe the others out, or something similiar. Don't give any broad hints to your players. The migration should make perfect sense if you know everything that's going on in the depths, but should be unfathomable to those higher up. Let them deal with the effect, and, if they're curious, have them go questing for the cause. For instance, have one group start encroaching on the territory of a city your players have contact with, and have the city hire them to find out where they're coming from and how many more might be on their way.
The Guide is no more specific than Underdark, so the OP needs to ask Ed over at Candlekeep what he thinks about the topic. Then again, Underdark is specific in saying that the domains described therein are "jsut" those known to the sages and people of the surface in one way or another. It is by no means an exhaustive list and "only" covers the Underdark beneath Faerûn, i.e. anything between the Sword Coast and the Hordelands.
The OP was, but primarily because they want to put new races into the campaign, and was looking for a way for them to show up without unduly leading to "so why hasn't anyone seen these guys before?" That's not really something Ed would handle, though the OP is, of course, welcome to ask.
What Ed should be able to handle are the OP's questions ...
How far does the Underdark extend?
Does it go all around the planet,
...
does it go down to the core,
...
is it possible to enter the Underdark in the Northlands and come up in Maztica?
... while Underdark already answers the last one:
Or is the map in the book "The Underdark" the only known Underdark on the planet?
... there are no other maps, but there sure is more Underdark beneath FAerûn than on the existing maps.
The kid was asking a question he simply did not know the answer to, I don't know why you feel the need for the sarcastic "...." replies. Completely irrelevant Zanan.
Dear fellow poster!
The OP asked for specific details on - as far as I see it - Faerûn's Underdark. Above, I stated who is the person to ask, i.e. the world's designer. Why? Because outside Underdark and DdUGttUD, there is no further printed information. As Thinblade was, IMHO, under the false impression that the OP was asking about the Underdark in general, I pointed out those questions of the OP which Ed Greenwood - as Faerûn's father - was or should be well capable of answering. To split these questions, I used " . . . ", though I could have used " + + + " or a couple of words or what have you. Nothing more, nothing less. If I want to be sarcastic, rest assured, I will use words.
NB: BTW, I wonder how you can define Thinblade's age by his / her name.
I have no clue whatsoever whether the Underdark of "Greyhawk the setting" has ever been dealt with ... outside the big Spider Queen adventures. And these are not even a tenths as revealing as the FR books on the matter.
Of 3rd party books, most are concerned with races rather than the Realms Below. The reason more often than not is quite simple: they want that their products shall be used in D&D settings and if they whip up "Underdarks" of their own, conflicting stuff may appear and deter DMs from buying the stuff. So while you find tons of useful information, the place as such usually remains open to your imagination.
Dear fellow poster!
The OP asked for specific details on - as far as I see it - Faerûn's Underdark. Above, I stated who is the person to ask, i.e. the world's designer. Why? Because outside Underdark and DdUGttUD, there is no further printed information. As Thinblade was, IMHO, under the false impression that the OP was asking about the Underdark in general, I pointed out those questions of the OP which Ed Greenwood - as Faerûn's father - was or should be well capable of answering. To split these questions, I used " . . . ", though I could have used " + + + " or a couple of words or what have you. Nothing more, nothing less. If I want to be sarcastic, rest assured, I will use words.
NB: BTW, I wonder how you can define Thinblade's age by his / her name.
That is no justification, but nice try. And btw, if you want to use words for sarcasim, or your indirect sarcasim ".......". I can assure you, that you will not enjoy it with me. But please, as you will.
I, the OP, am satisfied, given Zanan's knowledge of the FR drow, that I have only to research the Maztica connection. I am aware of some drow showing up in the Hordelands, but their mention is pretty scant, and they seem to be non-existent in Kara-Tur and Al Qadim, according to what I've read this afternoon. (Al Qadim would make sense. Too damned much sun.)
However you work it out, just have the class to do it in private. I hate it when people feel they need to air their dirty laundry in public.
I hate it when people get involved in situations that do not protain to them. But I implore you to join in if you feel the need to "Referee" this already forgotten dispute.
Shiftykitty, I seem to remember that The Grand History had a few bits and pieces on Maztica, but I am sure people on here will know too. The Forgotten Realms Atlas CD-ROM featured one of the island temple cities, though I cannot tell whether they were placed in the Realms Below.
Shifty ... neither the Maztica CS, nor Fires of Zatal, nor Endless Armies features anything on the Underdark. With regards to drow and other subterran races, page 26 of the Maztica CS says:
CAVES OF FAR PAYIT:
The peninsula is based upon a bed of very porous limestone, and consequently the caves that have formed there are continuously growing and expanding. Many large cave systems have come together, to form a veritable highway net of passageways underneath the jungled lands.
Many of these are air-filled caves, while a roughly equal number are submerged. They are host to a variety of subterranean life, including fish, bats, molds, and fungi; and also to a few monstrous
denizens of the dark who are not used to encountering human intruders.
There are no intelligent races living in these caves, however, although they seem tailor-made for
a dwarf, gnome, or even goblin or drow homeland. When they are discovered by a member of such a race, word of their existence will spread across the Realms. It is likely that they will become settled as a 'new frontier' that has opened before the crowded populations of the underground peoples of the Forgotten Realms.
It is not known whether or not these caverns offer any connections to subterranean regions beyond Far Payit itself. Such links cannot be ruled out, however.
Kara-tur is equally devoid of Underdark or drow information. There might be, however, Dragon articles on this.
Maztica needs to have at least a mini-Underdark, since the dwarves managed to travel under a good portion of the continent to emerge in the middle of the deserts. The problem there would be surface connections. There probably aren't very many, and now that the passage back to Faerun is closed, that's one less way to get in.
Maztica's drow are dealt with, I think, primarily in the last bit of Book 2 and Book 3 of the Maztica novel trilogy. I haven't read book 3, though, and it's been years since I read book 2, but IIRC, they all get turned into driders by a vengeful Lloth.
Since the modern drow diaspora started from what is now the Great Rift, it would make sense that they would spread east and south (leading to Kara-Tur and Zakhara), in addition to north and west. However, despite being logical, there's never been any sources to back this up. That's your opening, though, if you want it. And since there probably hasn't been any contact with their more "traditional drow" cousins, there's no telling how they might have changed during the passing millennia.
http://www.aeolia.net/dragondex/
Do a search for drow, or underdark, or anything else that might be of help, and at least narrow it down to specific issues.
And let us know if you have more questions, we'll do our best to answer.
I agree that a manual search of Dragon would not be worth it. However, it might be worth it to take a look through the DragonDex:
http://www.aeolia.net/dragondex/
-An amazing link. Thank you for that.