Definitions from Wiktionary (Underground)

▸ adjective:  (not comparable) Below the ground; below the surface of the Earth.
▸ adjective:  (figurative) Hidden, furtive, secretive.
▸ adjective:  (of music, art etc.) Outside the mainstream, especially unofficial and hidden from the authorities.
▸ adverb:  Below the ground.
▸ adverb:  Secretly.
▸ noun:  (geography) Regions beneath the surface of the earth, both natural (eg. caves) and man-made (eg. mines).
▸ noun:  (chiefly British) Synonym of subway: a railway that is under the ground.
▸ noun:  (with definite article) A movement or organisation of people who resist political convention.
▸ noun:  (with definite article) A movement or organisation of people who resist artistic convention.
▸ verb:  To route electricity distribution cables underground.
▸ noun:  (UK, rail transport) The London Underground.
▸ Also see underground

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