Is there an idiom for something that will never happen?


Inspired by [this question](https://english.stackexchange.com/q/436987) on
English SE, and the [List of idioms of
improbability](https://en.wikipedia.org/wiki/List_of_idioms_of_improbability)
in Wikipedia which does not have an entry for Japanese, I was wondering... Is
there an idiom in Japanese for something that will never happen?




This one is not strictly 'never', but it does indicate very low probability:

瓢箪から駒 (ひょうたん から こま) a horse from a gourd

