netherworld

Short definition from Wiktionary

Noun

1. 1
   The place to which one's spirit descends upon death, conceived as below the surface of the earth.
2. 2
   The locale of the spirit world or afterlife, whether deemed to be situated below the world of the living or not.
3. 3
   Specifically, a location of punishment in the afterlife; a hell.
4. 4
   A hidden, shadowy, or sinister subculture, such as that of organized crime.