I am working on some Darklands/Underdark material for Dark Sun. My starting point is known existing ones in canon.
Subterranean. There are many “dungeons” which are not underground. I will make exception for something like inside a mountain, but it should be underground.
There is a difference between extensive underground dungeon and an Underdark. The Underdark should be able to support some kind of ecosystem (not necessarily a natural one) and often at least one society.
These are the ones I can think of for sure.
Everything beneath Guistenal
Hidden gith city beneath the Black Spine Mountains
The mindhome/dark spider caverns near Cedrilte (from Shattered Lands).
UnderTyr and iron mines of Tyr.
There are certainly others. These below are potentials.
Exposed cave system after the Great Earthquake
Mines of Raam
I am sure there are others. Any others I am missing? I do enjoy creating new ones, but I’d like to be sure I’ve got all the extant examples first.
There’s the under the obsidian plain to consider, there’s a route to the gate to the paraelemental plane of obsidian… could have a bunch of the undead scarlet weavers, odd elementals, hej-kin, pech or other modified elementals about. Pockets of zhen that haven’t been released yet, etc…
The Lake of Golden Dream seems to have some underdark possibilities.
"At the foot of The Smoking Crown lies a strange wonderland of odd rock formations, boiling pools, and bubbling mud, eventually giving way to a steaming lake several miles wide. The Lake of Golden Dreams is the most visited location in the area, for the vapors rising from the scalding water are said to grant visions. Mystics, seers, and lunatics make their homes along the lake’s shore, rotting their minds by sampling the unhealthy mists.
In addition, the lake might conceal a submerged city. Where the blistering water is not too deep or cloudy, one can see remnants of old walls on the bottom, with dark cave mouths leading deeper into the earth."
Though not an official published product, there is Reevt Canyon from the Dead Land of Athas Net project.
“Reevt Canyon was once normal, flat, ground. Underneath this ground laid the many homes of the underdark gnomes. Their enormous underground complex weakened the structural integrity of the ground above. When the Obsidian Tide came, the ground caved in forming what is now Reevt Canyon. Reevt Canyon is home to the multitude of undead who died when the ground fell from under them, the underdark gnomes who died there before the Obsidian Tide during the Cleansing Wars and the human soldiers that died during that same period. The canyon is twelve miles wide in parts, and up to 3,000 feet deep. More undead dwell here than any other place Athas, which makes it a very dangerous place indeed. Legend has it that a powerful artifact was lost when the ground caved in, an artifact whose power could possibly rid Athas of all its undead.”
Isn’t there an cavern complex underneath Cleft Rock Oasis near the Pristine Tower? Sadira encounters Dhojakht there and discovers a number of halflings (presumed Rhulisti) in suspended animation. Sounds like a Nature Master redoubt to me.