Definition of netherworlds:
(n) :
The place to which one's spirit descends upon death, conceived as below the surface of the earth.
(n) :
The locale of the spirit world or afterlife, whether deemed to be situated below the world of the living or not.
(n) :
Specifically, a location of punishment in the afterlife; a hell.
(n) :
(by extension) A hidden, shadowy, or sinister subculture, such as that of organized crime.