underground

1 undertaken or done so as to escape being observed or known by others an underground operation to smuggle ancient artifacts out of the country Synonyms backstairs, behind-the-scenes, clandestine, covert, furtive, hole-and-corner, hugger-mugger, hush-hush, private, privy, sneak, sneaking, sneaky, stealth, stealthy, surreptitious, undercover, secret, underhand, underhandedRelated Words back-channel, closed-door, off-the-books, off-the-record; classified, confidential, restricted, top secret, undisclosed; concealed, hidden, secreted, subterranean, unadvertised, unexposedNear Antonyms acknowledged, avowed; aboveboard, straightforward, unconcealed, undisguised; unclassified, unrestricted; clear, evident, manifest, obvious, patent, plainAntonyms open, overt, public

2 being, situated, or operating beneath the surface of the earth an underground shopping mall that is part of the city's subway system Synonyms subsurface, subterranean, belowgroundAntonyms aboveground, surface