underground

US /ˈʌn.dɚ.ɡraʊnd/
UK /ˈʌn.dɚ.ɡraʊnd/
"underground" picture
1.

beneath the surface of the earth

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The miners work underground.
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Many animals live underground.
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above groundoverground
2.

in secret; surreptitiously

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They operated an underground network to help refugees.
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The band gained a large underground following before hitting the mainstream.
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secretclandestinecovertsubterraneanhidden
1.

a system of underground trains; a subway or metro

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I take the Underground to work every day.
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The London Underground is also known as the Tube.
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subwaymetrotube
2.

a secret group or movement, especially one resisting an occupying power or a totalitarian government

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The resistance movement operated as an underground.
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They joined the underground to fight for freedom.
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resistanceinsurgencyclandestine movement
1.

situated or operating beneath the surface of the earth

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The city has an extensive underground railway system.
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They discovered an underground cave.
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above-groundoverground
2.

denoting or relating to a cultural movement or subculture that is not mainstream or commercial

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He's a fan of underground music.
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The film has an underground following.
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alternativeavant-gardenon-mainstreamsubcultural