The Underdark is the vast network of underground caverns and tunnels underneath the surface of Aldrazar. It is home to a host of evil beings driven deep into the caverns at the end of the Conflict Wars.
World Above - The surface area of Aldrazar, The name those who live in the Underdark give the surface.
Upperdark - The first three miles below the surface. It is here where the surface dwellers and those in the Underdark most often meet.
Middledark - Located three to ten miles below the surface, this layer is where most of the Underdark cities are located.
Lowerdark - Ten miles or more below the surface, the Lowerdark is where even those who know the Underdark loath to go.
The Underdark is extremely dangerous, especially to those not native to it. There are 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 have developed darkvision. Food can be extremely difficult to find, and much of the natural vegetation is poisonous.
The Underdark is not one giant cavern under Faerun, but rather, many giant 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 Underdark is divided into several domains that are similar to continents of the world above. While it is possible to travel from one place to another within a domain, separate domains tend 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.
The Underdark's underground landscape varies from small subterranean fissures almost impossible for adventurers to crawl through to wide expanses of deep water that hide coral caves, hollowed out sections of ice in which creatures live, and places where fungus, bones, or even pure force form "caves."
This varied landscape is as dynamic as the surface of Aldrazar. Some areas are even ever changing over the course of time by underground magma rivers or the emptying of underground lakes due to cracks in the rock formed by earthquakes. This means maps of the Underdark are highly unreliable.
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 need to be well prepared.
The Underdark goes 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 for the strong-hearted. The nerve-wracking silence makes even experienced adventurers grow agitated after a few days.
Just like deep dungeons, the air can grow stale and even poisonous in the Underdark. It can contain explosive gases from underground volcanic activity or other deadly gases bubbling out of deep fissures in the rock. Bad air and poisonous fumes can collect in portions of tunnels and caves more or less isolated from the main vaults and caverns. Siphon-like tunnels holding water can also trap these gases for a long time.
The temperature underground varies according to depth and the proximity of thermic activities in the area. Throughout the year the temperature remains stable and moderate in most regions, usually slightly clammy or chilly. The water underground is extremely cold and poses a hypothermia risk.
Plant life and animal life can be as varied underground as on the surface. Ranging from small insects to hook horrors or black dragons which have set up their realms deep down, everything can be expected. Plant life is unable to use sunlight as a source for energy, and thus takes on strange forms adapted to life underground. Many fungi are also found underground. Being unable to digest normal nutrients most of the time, they rely on absorbing faerzress for their energy.