The Underdark

Caverns larger than cities yawn across forests of mushrooms. Bioluminescent mosses, crystals, and even animals illuminate miles of caves and tunnels.

The geology of the known world is riddled with vast cave systems, rumored to be home to rivers, waterfalls, and enormous seas of cold, fresh water. Underground, the temperature is always 50 degrees, though it gets warmer the deeper you go. Caverns larger than cities yawn across forests of mushrooms. Bioluminescent mosses, crystals, and even animals illuminate miles of caves and tunnels. Sometimes (along with Rashael called the Twilight Kingdoms due to both the pleasant temperature and occasional multicolored perpetual light, lush vegetation is grazed by blind, albino animals who rely on scent rather than sight in a world that is absolute darkness if you wander too far.
For those creatures living in the islands of perpetual twilight, the colors of both plants and animals is astonishing. Those who live in darkness tend to extremes of pale albino white for creatures who never leave the dark, and jet black to better hide when traveling through the twilight. Sensitive to sunlight, they universally see in the dark (unless blind).
While many of the Underdark’s residents would be dangerous were they to escape to the surface, two things keep them underground: a) the fact that many evolved there and don’t want to go the surface, and b) the buffer provided by the dwarves, who long ago colonized the larger access points and built their cities there. The dwarven kingdoms closed their doors to the surface millennia ago, and the only dwarves anyone has had contact with are the rare dwarven community descended from those who stayed on the surface.

Rare on the surface, its is said great dwarven cities are built underground at the major entrances to the Underdark.