Underdark
The underdark is as varied as the realms above it, from caverns so large one forgets they have a ceiling, to deep, lightless lakes of eyeless fish and twisted magic.
Geography
The only feature all areas of this massive Ream share is being subterranean. From forests of fungi to seas of dark water, the underdark hosts as many landscapes as can exist without a sky. Stalagmites large enough to compare to mountains, craters and chasms that provide fast but risky access to the realm's lowest depths, fields of mosslike lichens, and of course caves and tunnels of all description. To simply call it a cave system is a vast oversimplification, and, among other faults, ignores the broad stretches of undeniably man-made dungeon manifested in the stone.
Ecosystem
One quirk of the underdark's ecosystem which differentiates it from normal cavernous areas is its inclusion of adventurers specifically in the life cycle of so many native species. Sapient humanoids who seek after treasure are targeted specifically by many flora and fauna of the lightless lands, and serve as necessary parts of the ecosystem, both as a source of surface-specific nutrients, as well as a transport of spores and other material between regions.
Localized Phenomena
As a realm which literally undercuts all other Realms, the Underdark is uniquely subjective to the Realm Magic of several places. The underdark of the Beastlands, for instance, shares the blessing of sapience in its animal life.
Climate
Being entirely subterranean, the underdark does not experience weather or day/night cycles.
Fauna & Flora
The presence of certain flora and fauna is often used as a benchmark for when one has ceased to be in a deep cave and has entered the Underdark. In addition to the mosses, ferns, algae, molds, and mushrooms common to most cave systems, the underdark is host to a multitude of uniquely indigenous flora, particularly fungi, including Barrelstalk, Ripplebark, and Darkreed.
In addition to bats and the usual burrowing creatures such as xorns, bulettes, and earth elementals, the Underdark is home to several species evolved specifically for its permanently darkened and adventurer-rich environement. Many have migrated or been transported to other realms, but Mimics, Cloakers, Gricks, and even several species of Ooze such as the grey ooze and black pudding, trace their roots to the Underdark.
Natural Resources
The Underdark is host to a wide variety of produce that cannot be cultivated in other realms, but owing in part to its perishable nature, and in part to the dietary differences between realm denizens, it isn't very popular on the surface. Mineral exports are more profitable, though the transport of heavy ores to the surface is a difficult and costly venture, especially given the underdark's popularity as a haven for thieves and outlaws.
Tourism
The Underdark's population is more broadly scattered, boasting far fewer cities than the surface, but is just as wealthy in landmarks and objects of wanderlust.
Alternative Name(s)
The Realms Below, The Lightless Lands
Type
Underground / Subterranean
Location under
Included Locations
Inhabiting Species