| Main Entry: | |
| Part of Speech: | adjective |
| Definition: | below the surface |
| Synonyms: | below ground, buried, covered, in the recesses, subterranean, subterrestrial, sunken, underfoot |
| Antonyms: | aboveground, ground, sky |
| Main Entry: | black market |
| Part of Speech: | noun |
| Definition: | illegal commerce |
| Synonyms: | bootleg market, gray market, illegal sales, illegitimate business, illicit business, run, shady dealings, underground, underground market, underworld market |
| Main Entry: | clandestine |
| Part of Speech: | adjective |
| Definition: | secret, sly |
| Synonyms: | artful, cloak-and-dagger, closet, concealed, covert, foxy, fraudulent, furtive, hidden, hush-hush, illegitimate, illicit, in holes and corners, on the Q. T., on the quiet, private, sneaky, stealthy, surreptitious, under wraps, under-the-counter, undercover, underground, underhand |
| Antonyms: | aboveboard, forthright, open, truthful |
| Main Entry: | deep |
| Part of Speech: | adjective |
| Definition: | extending very far, usually down |
| Synonyms: | abysmal, abyssal, below, beneath, bottomless, broad, buried, deep-seated, distant, downreaching, far, fathomless, immersed, inmost, low, profound, rooted, subaqueous, submarine, submerged, subterranean, sunk, underground, unfathomable, wide, yawning |
| Antonyms: | shallow |
| Main Entry: | hidden |
| Part of Speech: | adjective |
| Definition: | unseen, secret |
| Synonyms: | QT, abstruse, buried, clandestine, cloaked, close, clouded, concealed, covered, covert, cryptic, dark, disguised, eclipsed, esoteric, hermetic, hermetical, imperceivable, in the dark, indiscernible, invisible, latent, masked, mysterious, mystic, mystical, obscure, occult, out of view, private, recondite, screened, secluded, sequestered, shadowy, shrouded, surreptitious, ulterior, undercover, underground, undetected, undisclosed, unexposed, unknown, unrevealed, veiled, withheld |
| Antonyms: | bare, exhibited, exposed, open, out, seen, showing, uncovered |
| Main Entry: | secret |
| Part of Speech: | adjective |
| Definition: | hidden, unrevealed |
| Synonyms: | abstruse, ambiguous, arcane, backdoor, camouflaged, classified, cloak-and-dagger, close, closet, clouded, conspiratorial, covered, covert, cryptic, dark, deep, disguised, enigmatical, esoteric, furtive, hush-hush, mysterious, mystic, mystical, obscure, occult, on the QT, out-of-the-way, private, recondite, reticent, retired, secluded, shrouded, strange, under wraps, undercover, underground, undisclosed, unenlightened, unfrequented, unintelligible, unknown, unpublished, unseen, veiled |
| Antonyms: | clear, defined, explicit, forthright, honest, open, public, revealed, unconcealed |
| Main Entry: | subversive |
| Part of Speech: | adjective |
| Definition: | rebellious, destructive |
| Synonyms: | incendiary, inflammatory, insurgent, insurrectionary, overthrowing, perversive, riotous, ruinous, seditious, treasonous, underground, undermining |
| Antonyms: | loyal, obedient |
| Main Entry: | undercover |
| Part of Speech: | adjective |
| Definition: | secret, spy |
| Synonyms: | clandestine, concealed, confidential, covert, creep, furtive, hidden, hole-and-corner, hush-hush, incognito*, intelligence, on the QT, private, stealth, stealthy, sub-rosa, surreptitious, under wraps, underground, underhand, underneath |
| Antonyms: | known, open, public |
| Main Entry: | subway |
| Part of Speech: | noun |
| Definition: | underground railroad |
| Synonyms: | chute, metro, tube, underground |