The Unterrealm

"Take a long look at the sun, boys, for this may be the last you see of it for a very long time. No light, nor good down there below. Mite bit of coin, though. If we're lucky."

Beneath the surface of the Realm lies another land shrouded in darkness. An extensive network of caverns, chambers, and tunnels that form a world unto themselves. Known by many names, this underground world is commonly called the Underdark, or more formally, the Unterrealm.
There are numerous cities and settlements that dot the jagged landscape below, home to Duergar, Svirfneblin, and most notably, the Drow.