The Underdark

Usual depths and corresponding names

World Above; The name those who live in the Underdark gave the surface.
Upperdark; The first 5 kilometers below the surface. It is here where the surface dwellers and those in the Underdark most often meet.
Middledark; Located 5 to 16 kilometers below the surface, this layer is where most of the Underdark cities are located.
Lowerdark; 16 kilometers or more below the surface, the Lowerdark is where even those who know the Underdark loath to go.  

Domains

The Underdark is not one giant cavern under the world on the surface, 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 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 live, and places where fungus, bones, or even pure force formed "caves".   This varied landscape is as dynamic as the surface. 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 maps of the Underdark were highly unreliable.   People say that the underdark is deep below, however it appears that shifts in the earth have been pushing the underdark up to the service in some regions, which however aren't commonly known.  

Environment

The Underdark's environment varies 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 is mainly without any source of light save for the occasional luminous rocks or fungi. 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 makes traveling something only for the strong-hearted. The nerve-wracking silence can make 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 varies according to depth and the proximity of thermal activities in the area. Throughout the year, the temperature remaines stable and moderate in most regions, usually slightly clammy or chilly. The water underground is extremely cold and posed a hypothermia risk.  

Inhabitants

The Underdark is extremely dangerous, especially to those not native to it. There is the usual dangers associated with caverns: claustrophobia, occasional poor air circulation, and getting lost. There is no light except for occasional patches of fluorescent fungi; most Underdark inhabitants either have 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 is also the home of many predators and often-hostile races, including aboleth, beholders, cloakers, derro, drow, duergar, illithids, kuo-toa, driders, and svirfneblin.  

Ecology

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 is however unable to utilize sunlight as a source for energy, and thus took on strange forms adapting 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.   Source: Forgottenrealms