abstruseness, brainteaser, braintwister, charade, chiller, cliffhanger, closed book, conundrum, crux, cryptogram, difficulty, enigma, grabber, inscrutability, mindboggler, mystification, occult, oracle, perplexity, poser, problem, puzzlement, question, rebus, riddle, rune, secrecy, sixty-four-thousand-dollar question, sphinx, stickler, stumper, subtlety, teaser, thriller, tough nut to crack, twister, whodunit, why*

.................................mystery