Essentially, in order to hide something that could end the world- it would have to hide the existence of whatever it was that was going to cause it.

If it's astronomical- that means silencing literally millions and millions of people. Impossible. Colleges and Universities and professional and amateur astronomers... Too many people are looking at the sky.

Geological- well then you have the same problem.

There are just way too many independents to shut up.

I would scale it at a very low end of zero.