American English

Definition of underground adjective from the Oxford Advanced American Dictionary

      

    underground

     adjective
    adjective
    NAmE//ˈʌndərˌɡraʊnd//
     
    [only before noun]
     
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  1. 1under the surface of the ground underground passages/caves/streams underground cables
  2. 2operating secretly and often illegally, especially against a government an underground resistance movement
See the Oxford Advanced Learner's Dictionary entry: underground