The Underdark is a hostile environment buried under the surface of the world. It is divided into sections which may or may not connect in some way shape or form.
Upperdark: Extends from the surface to about 3 miles depth. Knowledge (dungeoneering) DC 20
Surving in the Upperdark:
Food - small animals and plant life are readily found - Normal survival checks
Water - many lakes, rivers, and water sources (many of which can be guarded)- Normal survival checks
Light - luminescent flora can give occasional light sources, but not enough to support those without some way of seeing in the dark.
Air is available due to many air vents so it is not an issue.
Typical Denizens:
Chitines
Drow
Dwarves
Goblins
Quaggoths
Minotaurs
Orcs
Svirfneblin
Stone Giants
Troglodytes
Wererats
Middledark: Starts at 3 miles depth an extends to 10 miles depth. Knowledge (dungeoneering) DC 25
Surviving in the Middledark:
Food can be difficult to find - Survival checks are made at -5.
Water is scarce and is often heavily guarded.
Light - luminescent flora is scarce in the wild, but can be prevalent in settlements, there is not enough to support those without some way of seeing in the dark.
Air is stale and there can be occasional poisonous pockets of air.
Typical Denizens:
Drow
Duergar
Fomorians
Grells
Grimlocks
Hook Horrors
Kuo-Toas
Maurs
Mind Flayers
Orogs
Umber Hulks
Lowerdark: Starts at 10 miles depth and extends below. Knowledge (dungeoneering) DC 30
Surviving in the Lowerdark:
Food cannot be found unless creatures resort to cannibalism - Survival checks are made at -10.
Water is almost non-existent and is heavily guarded.
Light - There are no light sources here and the darkness appears to attack natural light sources often extinguishing them and magical light sources draw out attackers. Those with darkvision can see their vision flicker at the edges of their sight.
Air is very stale and often there can be occasional poisonous pockets of air.
Typical Denizens:
Aboleths
Avolakias
Beholders
Cloakers
Deepspawn
Derro
Desmodus
Destrachans
Elementals
Extraplanar Creatures
Psurlons
Tomb Tappers
Undead