cryptic, cryptical, deep, inscrutable, mysterious, mystifying

of an obscure nature; "the new insurance policy is written without cryptic or mysterious terms"; "a deep dark secret"; "the inscrutible workings of Providence"; "in its mysterious past it encompasses all the dim origins of life"- Rachel Carson; "rituals totally mystifying to visitors from other lands"

• inexplicable, incomprehensible — incapable of being explained or accounted for