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English definition of “underground”

underground

adjective, adverb [not gradable]  /ˌʌn·dərˈɡrɑʊnd/ us  
below the surface of the earth; below ground: an underground garage Something that is done underground is secret or hidden, usually because it is not traditional or is shocking or illegal: an underground newspaper Officials believe the sighting of the suspect may have forced him to go underground (= to become secret).
(Definition of underground adjective, adverb from the Cambridge Academic Content Dictionary © Cambridge University Press)
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