Underdark

The Underdark

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

Geography

World Above

The surface area of Toril. The name those who live in the Underdark give the surface.

Upperdark

The first three miles below the surface. It is here where surface dwellers and those in the Underdark most often meet.

Middledark

Located three to ten miles below the surface, this layer is where most Underdark cities are located.

Lowerdark

Ten miles or more below the surface, the Lowerdark is where even those who know the Underdark are loath to go.

Domains

The Underdark is not one giant cavern under Faerûn, 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 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
• 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 hide coral caves, hollowed out sections of ice in which creatures live, and places where fungus, bones, or even pure elemental forces form "caves." This varied landscape is as dynamic as the surface of Toril. Some areas are always changing over time due to the flowing of underground magma rivers or the emptying of underground lakes due to cracks in the rock formed by earthquakes. This ultimately means that maps of the Underdark are highly unreliable.

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 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 make traveling something only for the strongest of heart. 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.

Climate

The temperature underground varies according to depth and proximity to thermally active areas. 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.

Inhabitants

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, getting lost, and 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. In addition to these dangers, the Underdark is also the home of many predators and often-hostile races, including aboleth, beholders, cloakers, derro, drow, duergar, dwarves, illithids, kuo-toa, driders, and svirfneblin.

Ecology

Plant life and animal life can 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 can, and should, be expected. Plant life is unable to utilize 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.

Type

Underground / Subterranean