DRichards
The most unthinkable thing is the most unimaginable and then, since imagination is endless, it will be thought of as crazy and will destroy us all and then which will make everything unthinkable since you can't think of anything.
When the most unthinkable thing happens, it no longer becomes the most unthinkable thing and the second best will become the first and then it will be thought about and then it will cause a massive paradox which will kill us all or just make us go insane
The most unthinkable thing that can happen is that the sun that heats us all up will collapse, not become a black hole, suck us in and kill us all. This will happen in about a billion years but if we disrupt the sun then it will happen.