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English definition of “underground”

underground

adjective, adverb     /ˈʌndəɡraʊnd/
UNDER SURFACE B2 under the surface of the ground : underground caves an animal that lives undergroundGeology - general words
POLITICAL ACTIVITY Underground political activities are secret and illegal : an underground political organization Secrecy and privacyTaking action against people in power
(Definition of underground adjectiveadverb from the Cambridge Learner's Dictionary © Cambridge University Press)

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