Adjective underground has 3 senses
- belowground, underground - under the level of the ground; "belowground storage areas"; "underground caverns"
Antonym: overhead (indirect, via surface, subsurface)
Antonym: surface (indirect, via subsurface, overhead)
- clandestine, cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, on the quiet, secret, surreptitious, undercover, underground - 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"
Antonyms: overt, open (indirect, via covert)
- guerrilla, guerilla, underground, irregular - used of independent armed resistance forces; "guerrilla warfare"; "partisan forces"
Antonyms: nonpartisan, nonpartizan (indirect, via partisan)
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Adverbial underground has 2 senses
- underground - in or into hiding or secret operation; "the organization was driven underground"
- underground - beneath the surface of the earth; "water flowing underground"
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Noun underground has 2 senses
- underground, resistance - a secret group organized to overthrow a government or occupation force
--1 is a kind of revolutionary group
--1 has particulars: Maquis
- metro, subway, tube, underground - electric underground railway
--2 is a kind of railway, railroad, railroad line, railway line, railway system
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