Underdark

• World Above: The name that those who lived in the Underdark gave the surface area of Toril.
• Upperdark: The first 3 miles (4.8 kilometers) below the surface. It was here where the surface dwellers and those in the Underdark most often met.
• Middledark: Located 3​ to ​10 miles (4.8​ to ​16 kilometers) below the surface, this layer was where most of the Underdark cities were located.
• Lowerdark: 10 miles (16 kilometers) or more below the surface, the Lowerdark was where even those who knew the Underdark were loath to go.

Domains

The Underdark was not one giant cavern under Faerûn, 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.

The Underdark's underground landscape varied from small subterranean fissures almost impossible for adventurers to crawl through to more conventional caves and caverns. But there were also wide expanses of deep water in which coral caves grew, blocks of ice that creatures hollowed out and dwelt inside, and cave-like spaces within masses of fungus or bone or else supported by bubbles of magical force.

This varied landscape was as dynamic as the surface of Abeir-Toril. Some areas were 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 meant maps of the Underdark were highly unreliable.

Maztica

The Underdark burrowed deep under the Trackless Sea, stretching between the continents of Maztica and Faerûn. Ancient dwarven legend talked about a great ancient war between the stout folk and the Drow that raged in the depths underneath the Trackless Sea. The war was ended by drow magics that lit the tunnels with fires so hot they melted rock and flooded the portion of the Underdark that connected Dwarf(ves) and drow who resided in the Maztican Underdark with their eastern kin. In fact, the catastrophe was known as Rockfire and took place many dwarven generations before the "discovery" of Maztican continent by Amn. The calamity was caused by uncontrolled magics of the Darkfyre harnessed by the drow. It left Underdark beneath Maztica believed to be uninhabitable, filled with poisonous gas and molten rock. The dwarves who became cut off from Faerûn, with passing centuries became desert dwarves who lived in shallow caves close to the surface of the House of Tezca desert. They believed that the Rockfire completely exterminated the drow and the rest of the dwarves. However, that was proven to be a false belief by the mid–14th century DR. The Underdark passages between the two continents were believed to be lost legends, gone, and forgotten by inhabitants of Faerûn.

Laerakond

Main article: Abeiran Underdark

When the Spellplague brought the continent of Laerakond to Toril, it also brought a part of the Abeiran Underdark below it. It was different from the Underdark of Faerùn, but besides that it was inhabited by drow and mysterious monsters, little was known about this region.

The Underdark's environment varied vastly from the surface world, making adventuring through it a dangerous expedition. Just like when trekking through a desert, adventurers traversing the Underdark's passages needed to be well prepared.

The Underdark was mainly without any source of light save for the occasional luminous rocks or fungi. Large portions of it were 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 made even experienced adventurers grow agitated.

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.

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.

The Underdark was inhabited by many species, including Unknowns, Beholders, Cloakers, Derro, Drow, Duergar, Dwarf(ves), Illithids, Kuo-Toa, Quaggoths, Rock Gnomes, and Unknown

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. Anything could be expected. Plant life was unable to utilize sunlight as a source for energy, and thus took on strange forms adapted 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. The basis of the Underdark food chain were Magivores: coral-like organisms that likewise fed off faerzress.

Some dangerous flora of the Underdark were the exploding Unknowns.