Definition of underground adverb from the Oxford Advanced Learner's Dictionary

      

    underground

     adverb
    adverb
    BrE BrE//ˌʌndəˈɡraʊnd//
     
    ; NAmE NAmE//ˌʌndərˈɡraʊnd//
     
    Terrorism
     
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  1. 1  under the surface of the ground Rescuers found victims trapped several feet underground. toxic waste buried deep underground a network of pipelines running underground
  2. 2in or into a secret place in order to hide from the police, the government, etc. He went underground to avoid arrest. If the group is outlawed they will go underground. See related entries: Terrorism
See the Oxford Advanced American Dictionary entry: underground