I wouldn't classify this a "belief" because I'm fairly certain it's not actually true, put I have been pondering lately the idea behind quantum immortality: that since I've now come into existence and am aware of it, it's impossible for me to experience nonexistence again (death), no matter how low the probability of my survival is. For every reality where I could have died, there will always be one where I didn't, and that's the only one that I'll ever be aware of. Again, I'm pretty sure it's bogus, but it's something to think about, for sure.
I also absolutely do not believe in the idea of a future nuclear apocalypse. It's never going to happen. Isolated nuclear incidents MIGHT happen, but all out nuclear annihilation is impossible. I don't even fathom how people still think it's a possibility, let alone inevitable. The only countries that could pull it off are Russia and the US, and they aren't run by crazy idiots. Even crazy idiots wouldn't condemn the world to radioactive/fiery doom over petty wars. Hell, at this point I'd almost feel completely safe guaranteeing that no country will ever use a nuclear weapon in an attack on any other country, but then North Korea keeps me from going quite THAT far. But even they must realize that a nuclear attack is suicide, right?