WordNet sense 1 (of an obscure nature):

cryptic, deep, inscrutable, mystifying, cryptical

WordNet sense 2 (incapable of being explained or accounted for):

incomprehensible, inexplicable

WordNet sense 3 (having an import not apparent to the senses nor obvious to the intelligence; beyond ordinary understanding):

mystic, mystical, occult, secret, orphic

WordNet sense 4 (confined to and understandable by only an enlightened inner circle):

esoteric