The Underdark

The Underdark is the vast network of underground caverns and tunnels underneath the surface of Resanith. It is home to a host of evil beings deep within the endless caverns.

Geography

 * World Above
 * The surface area of Resanith. The name those who live in the Underdark gave 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 knew the Underdark refused to go.

Domains
The Underdark is not one giant cavern under Resanith, 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 was possible to travel from one place to another within a domain, separate domains tend to have very few passages linking them.

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, or hollowed out sections of ice in which creatures lived.

This varied landscape is as dynamic as the surface of Resanith. 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 are 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 need 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 devoid of 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 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 may 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 might 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 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, 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.

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, dwarves, illithids, kuo-toa, 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 be expected. Plant life is unable to utilize sunlight as a source for energy, and thus took on strange forms adapted to life underground. Many fungi are also found underground.