There aren't any personal-level events that strike me with an existential terror. As an adult male human of the large, imposing, American, hairy sub-types, I don't worry about physical violence, accidental death, accidental maiming, or much else. I don't worry about these because should they happen, I will not be the initiator and thus these events will be out of my control. There's nothing to worry about in events I can't control for what I think are obvious reasons, and need no explaining.
There are a few impersonal-level events that do strike with an existential terror, on a fairly regular basis and while I recognize the irrationality of this fear since the events will be far outside of my ability to help or control, I still think about them. Surviving a nuclear exchange is one such event, and I agree with my father's words to 8 year old me, "if there is going to be a nuclear war, I would prefer that you and your brothers not live to see the world after." That was an interesting trip to the guidance counselor after a duck and cover drill.
One of my favorite movies details the after effects of another sort of event that wakes me up at night. The simple premise is this: sometimes humans are too smart for their own good, and sometimes we experiment with shit that could result in a bad event, the sort of bad event that instantly causes us, our planet, or possibly even the universe to just go away. I'm smart enough to have gone further with my education but as a young genius I was unmotivated and that might be for the best because I'd probably treat the LHC as the biggest Nerf gun and mini black holes as foam darts. In other words, I'm smart enough to know exactly how crazy I am for worrying about this, but I'm still just a genius who worries about one or two things.
I was reading about some ideas some other smart people have for detecting the presence of other dimensions through the detection of mini black holes using the LHC and science to do sciencey things when the anxious voice in my head said, "this is how the universe ends, if you can bend or break a fundamental rule of the universe and then everything goes blip." I thought about this for a minute, my mouth full of coffee and my brain full of stupid. Then I remembered that the universe is really big. Like, really really really big. We know that the universe is big because look up at night, that just goes on for effectively forever. If that alone isn't enough to remind you that no matter how smart you are, you're still just a bag of meat on a rock, then you're probably a priest who is utterly confident of your place in some imaginary plan. The fucking arrogance of thought that the idea of a greater plan to life just irritates the ever-loving shit right of me. Seriously, if I'm constipated I'll just think about "God's plan for you" and then I'm destroying porcelain. That's the worst type of consolation, there's a plan for all suffering so that makes it ok because you're meant to experience life this way, starving orphan or crack-addicted newborn or mildly inconvenienced middle-aged person. I'll be right back.
Anyway, I was describing how I logiced my way out of anxiety. The point is that the universe is big, there millions or billions of galaxies, and those galaxies have lots of stars, and some of those stars must have planets, and some of those planets will have life, and some of that life will be multi-cellular, and some of that multi-cellular life will be investigating the universe just as we are. Since the universe is around 18 billion years old as far as we can tell, and our planet is only about 4 billion years old (suck it, creationists), and since there may be very old black holes which may or may not have started as stars, there exists the possibility that there is a much older civilization of intelligent enough creatures to build a Large Hadron Collider-equivalent. The universe is so vast, ooo you are so big, that this possibility is really more of a certainty. So my fears of some genius like me who wants to play Nerf tag with a particle accelerator and some micro black holes accidentally destroying this existence are stupid because if that were possible someone would have done it by now.
Now all I have to worry about is some sort of localized disruption of the time-space continuum rendering this section of time-space so wibbly-wobbly that we can not exist within it. That is much more comforting.
Of course the unlikeliest event of all would be the arrival of an alien super-intelligence and the co-opting of all life on this planet, but that's probably not going to happen and would be essentially the same as the events of the previous paragraph. I am so relieved.