[Idiom] when something is very improbable (strong disbelief that sth will happen)

[Meja]

Hello!

I know the idiom: I will eat my hat (if something happens or does not happen).

I would like to know if there are some other phrases or idioms which we can use when we want to say that something is very unlikely to happen and that we would be really surprised if that happened.

 

[Skrej]

'when Hell freezes over'
'when pigs fly'
'it'll be a cold day in Hell'
'when the sun rises in the west'
'when water runs uphill'
'once in a blue moon' (this is more of an expression of rarity, rather than disbelief)

 

[jwills241]

How about "not a snowball's chance in hell?"