The Underdark

The Underdark is an incredibly expansive network of caves and caverns that run beneath the whole of the planet; It being its own little world beneath the world. No one knows just how deep the Underdark goes but its known that it extends well beyond the deepest part of even the Dwarven Under Roads and that the deeper you go... The weirder and more terrifying (or spectacular) the things you encounter are.
The Underdark has its own unique, thriving ecosystem. Many of the flora and fauna have some form of bioluminescence or other. The Underdark is also home to a few empires of its own, with Drow Elves, Duergar (Dark Dwarves), and Deep Gnomes, hailing from its depths; with each having their own cities and ways of life. Most splendid and spectaular among them being the domain of the Drow Elves and their city of Menzoberranzan.