Definition of 'underground'

Noun

1. a secret group organized to overthrow a government or occupation force
      Synonyms: resistance
2. an electric railway operating below the surface of the ground (usually in a city); "in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'"
      Synonyms: metro tube subway system subway

Adjective

1. under the level of the ground; "belowground storage areas"; "underground caverns"
      Synonyms: belowground
2. conducted with or marked by hidden aims or methods; "clandestine intelligence operations"; "cloak-and-dagger activities behind enemy lines"; "hole-and-corner intrigue"; "secret missions"; "a secret agent"; "secret sales of arms"; "surreptitious mobilization of troops"; "an undercover investigation"; "underground resistance"
      Synonyms: clandestine cloak-and-dagger hole-and-corner hugger-mugger hush-hush secret surreptitious undercover

Adverb

1. in or into hiding or secret operation; "the organization was driven underground"
2. beneath the surface of the earth; "water flowing underground"

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