able, arcane,
cabalistic, censored, classified, close, closed, concealed, cryptic, dark,
enigmatic, esoteric, hermetic, hidden, hush-hush, incalculable, incognizable, invisible, latent, mysterious, occult,
puzzling, restricted, sealed, secret, smothered, stifled, strange, suppressed,
top secret, ulterior, unapparent, unapprehended,
unascertained, unbeknown, unbreatheable, uncharted,
unclassified, under security, under wraps, undetected, undisclosable,
undiscoverable, undiscovered, undivulgable, undivulged, unexplained, unexplored, unexposed, unfamiliar,
unfathomed, unheard, unheard-of, unidentified, uninvestigated,
unknowable, unknown, unperceived, unplumbed, unrevealable,
unrevealed, unseen, unsolved, unspied, unspoken,
unsuspected, untellable, untold, untouched, untraced, untracked, unutterable,
unuttered, unwhisperable, virgin