Underdark

The Underdark was the vast network of underground caverns and tunnels underneath the surface of the world, Abeir-Toril. It was home to a host of evil beings driven deep into the caverns at the end of the age of demons. It was split in to three depths: Upperdark, Middledark, and Lowerdark.

Geography

World Above

The surface area of Toril. The name those who lived in the Underdark gave the surface.

Upperdark

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

Middledark

Located three to ten miles below the surface, this layer was where most of the Underdark cities were located.

Lowerdark

Ten miles or more below the surface, the Lowerdark was where even those who knew the Underdark were loath to go

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 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 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.

Fauna & Flora

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 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, a magical energy found only in the Underdark, for their energy.