Underdark

The region called the Underdark is an entire world beneath the feet of the surface dwellers of Faerûn. Inhabited by monstrous and evil creatures that shun the daylight, the Underdark teems with entire cities and nations of derro, drow, duergar, and mind flayers (illithids). It is also home to even stranger races such as aboleths, beholders, and kuo-toas, as well as slaves of just about any intelligent humanoid found on the surface.
These evil beings battle or trade with each other for resources, magic, and power, forming alliances that collapse when plots unravel or better opportunities come along. Interspersed with the warring city-states are enclaves of gnomes, svirfneblin, dwarves, and other neutral and good folk, who remain isolated or resist encroachment by malign neighbors.
The three major powers of the Underdark are the dwarves, the duergar, and the drow. The dwarves have the advantage in that their realms often stretch into the Realm of Sky (the surface world), giving them ready access to food and water. The duergar are found in both the Shallows and the Deeps, while the drow are almost exclusively found in the Deeps, with outposts and small trading settlements in the Shallows.
Many other races have a foothold in the Underdark, the svirfneblin, orcs, goblins, kobolds, darklings, and derro establish settlements and fight for resources, but the drow and duergar have a stranglehold on large, sustainable sources of food and water, so the settlements of these other races tend to be smaller.

Set in Stone?

"I'm not crazy… I tell ya. I first noticed it on some of the expeditions we did in the Shallows. We were off the major routes, the known routes - and that's important - and I was mapping, such as it was. Over time, I realized that even though some of the passages we were taking were the same, the side caverns and side passages we didn't take were different.

"There was this fungus cave three miles into the passage. We had stopped there twice, but the third time we used the passage, it wasn't there. No sign of a cave-in, or rockslide, smooth stone where the cave once stood. Like it had never been there.

"We became pretty well-known, and a bit feared, enough that we were actually able to enter Menzoberranzan. The City of Spiders is surrounded by a mile-wide spiderweb of passages known as the Warrens. The drow scouts say that the Warrens shift constantly, again without rockslides or cave-ins, passages, caves, and the like appear and disappear.

"The scouts say that the better known or more traveled a passage was, the more likely it was to remain the same, but the lesser known twists and turns were always changing. Course, the drow could have been pulling my leg.

"Well, I got curious. So when we found ourselves in the Great Rift of the dwarves, I did some snooping in their libraries. Looking at the maps from different time periods, whole cities have shifted positions over time. On one of the oldest maps, the City of Spiders was located almost thirty miles from its current location, yet every stalactite and stalagmite in the vault stayed exactly the same. Main passages and routes in and out were the same as today, except they wound through different stone. Some passages were longer or shorter, but they connected the same known points. Other caverns and passages had changed, and the smallest passages had changed the most. And it was the same for other cities.

"I have this theory. It's alive, the whole damn thing, and it moves. No one inside it notices, kind of like being in a carriage or on a ship and feeling like you're not moving when you are. And when it moves, it changes. But see, the more people who know about a thing like a cavern or a tunnel, the less that thing changes. So things feel "set in stone", but they're not. I'm not crazy.. I tell ya."

Geography

Regions of the Underdark

The Underdark is divided into many sections and regions. Vertically, many scholars divide the realm into the Upperdark (close to the surface), the Middledark, and the Lowerdark.
The Upperdark, also known as the Shallows, is the most populated area. Proximity to the surface provides a ready supply of food and water. The Middledark and Lowerdark are often lumped together as "The Deeps". Rarely is the Lowerdark also refered to as the Utterdeep. In the Deeps, food and water are scarcer, and the drow and duergar are often the two groups in control of large sources of sustenance.
In Undercommon, the Upperdark is known as Nar-Voth, while the Middledark is known as Sekamina, and the Lowerdark is known as Orv.

The largest regions of the Underdark are listed below:

The Northdark (under the Savage Frontier and the northern Sword Coast) The Buried Realms (Under Anauroch) The Deep Wastes (under Cormyr, Sembia, the Moonsea and the Cold Lands) Deepearth is the portion of the Deep Wastes under the Bloodstone Lands) The Earthroot (Under Thay, Mulhorand, stretching north to under Narfell) Old Shanatar (under Tethyr, Amn, and Calimshan) The Darklands (under the Dragon Coast to Chesssenta) Great Bhaerynden (under the Shining South)

Alternative Name(s)

The Darklands