underground

adjective

1 the underground tank beneath the gas pumps exploded:

subterranean, subterrestrial, below ground, buried, sunken, lower-level, basement;

rare hypogean.

▷antonyms surface; overhead.

2 the underground organizations are designed to spread populist propaganda:

clandestine, secret, surreptitious, covert, undercover, private, confidential, closet, hole-and-corner, cloak-and-dagger, hugger-mugger, back-alley, backstair, stealthy, conspiratorial, concealed, hidden, shrouded;

sneaky, sly, underhand, shifty, furtive;

resistance, subversive, guerrilla, mercenary, revolutionary, rebellious, dissident, insurgent, insurrectionary, renegade, mutinous;

Military black;

informal hush-hush.

3 the New York underground art scene:

alternative, radical, revolutionary, unconventional, unorthodox, avant-garde, experimental, innovative, groundbreaking, pioneering, novel;

subversive.

adverb

1 the insects spend years underground as larvae:

below ground, below the surface, under the earth, in the earth.

2 the group went underground and issued calls for national resistance:

into hiding, into secrecy, into seclusion, undercover, behind closed doors, out of sight.

noun

1 British he took the underground back to his studio apartment:

metro;

British underground railway;

North American subway;

British, informal tube.

2 the information had been furnished by the French underground:

resistance movement, resistance, illegal opposition;

partisans, guerrillas, freedom fighters;

in France, , historical Maquis.