Synonyms and related words :
able, enigmatic, incalculable, incognizable, invisible, mysterious, puzzling, sealed, strange, unapparent, unapprehended, unascertained, unbeknown, uncharted, unclassified, undetected, undisclosed, undiscoverable, undiscovered, undivulged, unexplained, unexplored, unfamiliar, unfathomed, unheard, unheard-of, unidentified, uninvestigated, unknowable, unknown, unperceived, unplumbed, unrevealed, unseen, unsolved, unspied, unsuspected, untouched, untraced, untracked, virgin