underground - definition and synonyms

 
adverb 

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/ˌʌndə(r)ˈɡraʊnd/
  1. 1
    below the surface of the ground

    miners who work all day underground

    Many species of beetle live underground.

     Synonyms and related words
  2. 2
    in or to a secret place away from the police or other authorities

    Left-wing activists were driven underground after the coup.

    go underground:

    The police became suspicious and he had to go underground.