Underdark

The Underdark was the vast network of underground caverns and tunnels underneath the surface of Abeir-Toril. It was home to a host of evil beings driven deep into the caverns at the end of the age of demons.

Absence of light results in darkness. So, what results from the absence of Good? The Underdark is a place with very little of either.

— Mordenkainen

Inhabitants

The Underdark was extremely dangerous, especially to those not native to it. There were the usual dangers associated with caverns: claustrophobia, occasional poor air circulation, and getting lost. There was no light except for occasional patches of fluorescent fungi; most Underdark inhabitants either had highly developed senses other than sight or had developed darkvision. Food could be extremely difficult to find, and much of the natural vegetation was poisonous.
In addition to those dangers, the Underdark was also the home of many predators and often-hostile races, including aboleth, beholders, cloakers, derro, drow, duergar, dwarves, illithids, kuo-toa, driders, and svirfneblin.

Geography

World Above
The surface area of Toril. The name of those who lived in the Underdark gave the surface.
Upperdark
The first 3 miles (4.8 kilometers) below the surface. It was here where the surface dwellers and those in the Underdark most often met.
Middledark
Located 3​ to ​10 miles (4.8​ to ​16 kilometers) below the surface, this layer was where most of the Underdark cities were located.
Lowerdark
10 miles (16 kilometers) or more below the surface, the Lowerdark was where even those who knew the Underdark were loath to go.

Domains

The Underdark was not one giant cavern under Faerûn, but rather, many huge networks of caverns and caves. As a result, it was not always possible to travel from one end of the Underdark to the other. The Underdark was divided into several domains that were similar to continents of the world above. While it was possible to travel from one place to another within a domain, separate domains tended to have very few passages linking them. The major domains of the Underdark were the Buried Realms, the Darklands, the Deep Wastes, the Earthroot, the Glimmersea, Great Bhaerynden, the Northdark, and Old Shanatar.

Topography

The Underdark's underground landscape varied from small subterranean fissures almost impossible for adventurers to crawl through, to wide expanses of deep water that hid coral caves, hollowed out sections of ice in which creatures lived, and places where fungus, bones, or even pure force formed "caves."
This varied landscape was as dynamic as the surface of Abeir-Toril. Some areas were always changing over time because of underground magma rivers or the emptying of underground lakes due to cracks in the rock formed by earthquakes. This meant maps of the Underdark were highly unreliable.

Maztica

The Underdark burrowed deep under the Trackless Sea, stretching between the continents of Maztica and Faerûn. Ancient dwarven legend talked about a great ancient war between the stout folk and the drow that raged in the depths underneath the Trackless Sea. The war was ended by drow magics that lit the tunnels with fires so hot they melted rock and flooded the portion of the Underdark that connected dwarves and drow who resided in Maztican Underdark with their eastern kin.
The catastrophe was known as Rockfire took place many dwarven generations before the "discovery" of Maztican continent by Amn. The calamity was caused by uncontrolled magics of the Darkfyre harnessed by the drow. It left Underdark beneath Maztica believed to be uninhabitable, filled with poisonous gas and molten rock. The dwarves who became cut off from Faerûn, with passing centuries became desert dwarves who lived in shallow caves close to the surface of the House of Tezca desert. They believed that the Rockfire completely exterminated the drow and the rest of the dwarves. However that was proven to be a false belief by the mid-14th century DR. The Underdark passages between the two continents were believed to be lost legends, gone, and forgotten by inhabitants of Faerûn.

Ecosystem

The Underdark's environment varied vastly from the surface world, making adventuring through it a dangerous expedition. Just like trekking through a desert, adventurers traversing the Underdark's passages needed to be well prepared.
The Underdark was mainly without any source of light save for the occasional luminous rocks or fungi. Large portions of it were also without food or water, making well-stocked food supplies a must-have when exploring uncharted terrain underground.
Lack of light combined with a tomb-like silence made traveling something only for the strong-hearted. The nerve-wracking silence made even experienced adventurers grow agitated after a few days.
Just like deep dungeons, the air could grow stale and even poisonous in the Underdark. It could contain explosive gases from underground volcanic activity or other deadly gases bubbling out of deep fissures in the rock. Bad air and poisonous fumes could collect in portions of tunnels and caves more or less isolated from the main vaults and caverns. Siphon-like tunnels holding water could also trap these gases for a long time.

Climate

The temperature underground varied according to depth and the proximity of thermal activities in the area. Throughout the year, the temperature remained stable and moderate in most regions, usually slightly clammy or chilly. The water underground was extremely cold and posed a hypothermia risk.

Fauna & Flora

Plant life and animal life could be as varied underground as on the surface. Ranging from small insects to hook horrors or shadow dragons, which set up their realms deep down, everything could be expected. Plant life was unable to utilize sunlight as a source for energy, and thus took on strange forms adapted to life underground. Many fungi were also found underground. Being unable to digest normal nutrients most of the time, they relied on absorbing faerzress for their energy.
Some dangerous flora of the Underdark were the exploding bibberbangs.

Type

Region

Society

Races	Aboleth, cloaker, drow, duergar, illithid, kuo-toa, quaggoth, svirfneblin
Religions	Drow pantheon, Deep Duerra, others

Alignments	LG	NG	CG
	LN	N	CN
	LE	NE	CE

Commerce

Imports	Armor, food, slaves, timber, weapons
Exports	Armor, exotic goods, magic, weapons

Politics

Gov't type

City-states (autocracy, magocracy, matriarchy, monarchy, theocracy)