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English definition of “underground”

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underground

adjective, adverb uk   /ˌʌn.dəˈɡraʊnd/ us    /-dɚ-/

underground adjective, adverb (BELOW EARTH)

B2 below the surface of the Earth; below ground: an underground cave/passage/cable Moles live underground.
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underground adjective, adverb (SECRET)

An underground activity is secret and usually illegal: an underground newspaper/movement The Communist Party was forced (to go) underground, and its leaders went into hiding.

underground

noun uk   /ˈʌn.də.ɡraʊnd/ us    /-dɚ-/

underground noun (TRANSPORT)

the underground A2 [S] ( also the tube) UK a railway system in which electric trains travel along passages below ground: the London Underground They went on the underground.

underground noun (PEOPLE)

[U] people in a society who are trying new and often shocking or illegal ways of living or forms of art: In Britain and the USA in the 1970s, the underground was a powerful subversive force.the underground a group of people who secretly fight against the government: He was a member of the underground, harassing the invading army.
(Definition of underground from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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