Chinese puzzle
Gordian knot
arcanum
baffle
bafflement
baffling problem
bewilderment
bother
brain twister
cabala
classified information
confidence
confidential communication
confoundment
confusion
conundrum
crossword puzzle
crux
dark horse
dilemma
discomposure
disconcert
disconcertedness
disconcertion
disconcertment
disturbance
embarrassment
enigmatic question
esoterica
fix
floorer
frontier
frontiers of knowledge
guarded secret
hermetics
jam
jigsaw puzzle
knot
knotty point
matter of ignorance
mind-boggler
mystery
mystery of mysteries
mystification
n
node
nodus
nonplus
nut to crack
perplexity
problem
Enigma machine
a riddle wrapped up in an enigma
enigmatic
anachronism
anomaly
compulsion
inconsistency
intricacy
muddle
paradox
poser
puzzle
riddle
sigma
stigma