The Oxford Thesaurus An A-Z Dictionary of Synonyms - underworldn.

1) Usually, the underworld. organized crime, the syndicate, the Mafia, the mob, Cosa Nostra, criminals, the criminal element, Colloq gangland: Lorenzo was fast becoming a well-known figure in the underworld. 2) nether regions, abode of the dead, Hades, Hell, Avernus, Dis, Orcus, Facetious Egyptian underground: I think I prefer contemplating the Happy Hunting Ground of the American Indian to the underworlds of Christianity and classical Mediterranean culture.