unthinkable

unthinkable Show phonetics
adjective ━━ a. 考えられない; とてもありそうもない, 信じられない.
too shocking or unlikely to be imagined as possible:
You can't imagine what it would be like to have your child die - it's quite unthinkable.

the unthinkable
an unthinkable event or situation:
The unthinkable had happened - his secret activities had been discovered by the press.

(from Cambridge Advanced Learner's Dictionary)

M.C. Escher - The Graphic Work
Escher's Art. Presenting the structurally unthinkable as though it were a law of nature