The Underdark

The Underdark, a term used colloquially used for any and all of the space under the earth, isn't a nation on its own; but rather contains a multitude of nations, kingdoms, clans, tribes, beast, and cities. It is an unmapped maze of mines, caves, crevasses, tunnels, tombs, dungeons, canyons and other dark places under the ground that make up one enormous living, breathing, ecosystem in the damp dark of the earth.
Entire societies rise and fall in those dark caverns, many without ever having been seen or discovered by those who live above ground. Overworlders often intrude as interlopers, be they miners, colonists, adventurers, or invaders. Beneath the feet of the those above the ground, Goblin Kingdoms march to war against Drow Elves of Lolth; while Kobolds, normally so timid and fearful on the overworld boldly push back against the likes of Dwarves to form their own fiefdoms. Deep Gnomes reside in their secret cities, hiding from those who would do them harm and steal their riches. Fire Giants rule underground dark fortresses of obsidian, basalt, and black iron amidst rivers of lava. Cities of crystal, nestled in gargantuan spires of rock, floating on lava lakes, or built into the walls of caverns and canyons dot the underground landscape. It is a place of little light, wonders of creation, and profound darkness.
The Underdark is also spoken of in curses by those in the overworld. Myths and legends float freely about the evils under the ground, and for good reason. Legend has it that in the aftermath of the Cataclysm, many of the forces of darkness were driven under the ground into the Underdark where they choose to remain, sleep, or were imprisoned. The Dwarves know all too well the dangers found deep in the ground, for it was these creatures of darkness from the deepest depths, which for generations were the bane of their Kingdoms; collapsing their great network of underground highways, and tearing down their magnificent holds.

Domains

The Underdark is not one singular, giant cavern under Othlorias, but rather, many huge networks of caverns and caves. As a result, it is not always possible to travel from one end of the Underdark to the other. The few portions of it that have been mapped to any extent have been divided into several domains which are believed to share similar geographic features. While it's possible to traverse from one place to another within a domain, separate domains tended to have very few passages linking them. The major domains of the Underdark are: 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 varies 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 is as dynamic as the surface of Othlorias. Some areas are 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 means that maps of the Underdark are often highly unreliable.

Environment

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 majority of the Underdark is mainly without any source of light save for the occasional luminous rocks, fungi, plants or artificial light. Large portions of it are 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 makes even experienced adventurers grow agitated after a few days.
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.

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. But, many overworlders have made their homes in the reaches of the Underdark where cities are found, at least those amicable to their presence.
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, svirfneblin, and other dark monstrosities.