underworld
noun /ˈʌndəwɜːld/
  /ˈʌndərwɜːrld/
[singular]- the people and activities involved in crime in a particular place- the criminal underworld
- the Glasgow underworld
- the dark underworld of drug abuse and petty crime
- He ran an import-export business and had underworld contacts in Japan.
 Oxford Collocations Dictionaryadjective- dark
- murky
- seedy
- …
 - boss
- figure
- contact
- …
 - in the… underworld
 
- the underworld(in myths and legends, for example those of ancient Greece) the place under the earth where people are believed to go when they die- The three-headed dog Cerberus guarded the entrance to the underworld.