unknowable

unanswerable

ambiguous, arcane, cryptic, dark, deep, Delphic, double-edged, elliptical (or elliptic), enigmatic (also enigmatical), equivocal, fuliginous, inscrutable, murky, mysterious, mystic, nebulous, obscure, occult, opaque

abstruse, esoteric, recondite

cloaked, concealed, disguised, masked, shrouded

beclouded, bedimmed, befogged, clouded, cloudy, dim, faint, foggy, fuzzy, hazy, indistinct, indistinguishable, misty, muddy, obfuscatory, obnubilated, obscurant (or obscurantic), shaded, shadowlike, shadowy, sphinxlike

indefinite, inexact, inexplicit, noncommittal, questionable, unclear, uncertain, undefined, undetermined, vague

impenetrable, incomprehensible, indecipherable, inexplicable

eerie (also eery), uncanny, weird

impalpable, inappreciable, intangible, invisible

baffling, bewildering, confounding, confusing, mystifying, perplexing, puzzling, unfathomable

circuitous, indirect, roundabout

complex, complicated, difficult, obtuse