underworld Definition

1. mythol.
A world imagined to lie beneath the Earth's surface, the home of the souls of the dead.
2. The antipodes.

Thesaurus: hell, Hades, the nether world, the Inferno.
3. A hidden sphere of life or stratum of society, etc, especially the world of criminals and organized crime.

Thesaurus: organized crime, gangland, the Mafia, the mob, the syndicate, Cosa Nostra, criminals, riffraff, rackets.