underworld

Short definition from Wiktionary

Noun

1. 1
   The world of the dead, located underneath the world of the living; the afterlife.
2. 2
   That part of society that is engaged in crime or vice, and particularly those involved in organized crime.
3. 3
   The portion of a game that is set below ground.
4. 4
   Alternative letter-case form of underworld.