underground [ˈʌndɚˌgraʊnd]

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¹ underground adjective

• located or occurring below the surface of the earth
• secret and usually illegal
• of, relating to, or produced in a social and artistic world that is different and separate from the main part of society

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² underground adverb

• below the surface of the earth
• in or into a place that is hidden or secret : out of the view of the public

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³ underground noun

• a system of trains that run below the ground in a large city : subway
• a group of people who secretly work to oppose or overthrow a government