underground

nouns

a secret group organized to overthrow a government or occupation force
resistance, underground
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'"
metro, subway, subway system, tube, underground

adjectives

under the level of the ground
"belowground storage areas"; "underground caverns"
belowground, 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"
clandestine, cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, secret, surreptitious, undercover, underground

adverbs

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