"underground" in the noun sense
1. underground, resistance
a secret group organized to overthrow a government or occupation force
2. metro, tube, underground, subway system, subway
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'"
"underground" in the adjective sense
1. belowground, underground
under the level of the ground
"belowground storage areas"
"underground caverns"
2. clandestine, cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, 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"
"underground" in the adverb sense
1. underground
in or into hiding or secret operation
"the organization was driven underground"
2. underground
beneath the surface of the earth
"water flowing underground"
Source: WordNet® (An amazing lexical database of English)
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