10.

Gamma Ray Burst.

We're just sitting here doing our own thing and BAM! Earth gets sterilized in a lightspeed event. Zero warning.

The worst part is the people on the far side probably won't die instantly. Pretty quick since it would probably strip a big chunk of our atmosphere, but they'd still know the end was here.

Astramancer_

9.

The Strange Death of the Earth

Either a mishap at a supercollider or a hypervelocity ejection from a neutron star introduces a bit of strange matter to the planet. This bit of strange matter, or strangelet, starts converting the nucleus of every atom it touches into strange matter which creates a cascade effect through every bit of matter on earth turning our planet into a hot, lifeless ball of strange matter.

https://en.wikipedia.org/wiki/Strangelet

-primerush

8.

I think the Children of Men way of humanity going out is the most haunting. Just imagine nobody being able to make a baby, and the human race slowly dying off, knowing we can't do anything about it.

-Phil-Michaels

I agree with you but in more of a generalized sense. Whether it is a slow lingering end from a worldwide pandemic, environmental collapse or mass sterilization like your example, any apocalypse where we as a race would have ample time to realize "You know, we really aren't going to survive this" is haunting.

I almost prefer a gamma blast or something like that rather than watching humanity slowly burn out.

-Enzi42

7.

The old faithful geyser could shoot out a giant cloud of ash that would blot out the sun.

-awfulmspaint

You mean the Yellowstone Supervolcano?.....it's not just the geyser it's the whole entire park....the eruption could be comparable or even larger than the Toba Supervolcano which lowered the human population to 10k all over the world

-Enclair121

6.

Dust gets into a computer part.

Nuclear weapon systems are designed to 1) sense when enemy nukes are inbound, 2) sense when a nuclear weapon detonates, and 3) retaliate by launching nukes. Humans are involved too, but decisions are based upon the information from sensors and computer systems.

If a nuclear weapon sensor gives a false alarm and "detects inbound nukes" when there are none, that could lead to actual nukes being launched. Then those nukes will be sensed and cause more nukes to be launched.

Armageddon.

-Whooplash

Fun fact : that almost somewhat happened.

-maxdefolsch

5.

Highly contagious deadly virus impossible to cure.

-Gamelur84

4.

The craziest would be false vacuum collapse. It's a quantum field thing that postulates that all the empty space in the universe isn't actually true vacuum, but just a local minimum, and if a real vacuum, or global minimum, forms anywhere, it would spread at very nearly the speed of light, and destroy the entire universe and everything in it. This could already have happened, and we wouldn't know it until very shortly before the edge if it hit us. Fortunately, it's very unlikely to happen, even if our universe is one where it can happen. And there's no way to tell.

-Wadsworth_McStumpy

3.

Wave form collapse, essentially the possibility that some fundamental aspect of our reality relies on being in a superposition, observing it too hard could cause the waveform to collapse with results that we literally can't even begin to imagine. For comparison the next closest thing would be a false vacuum event which would change the very laws of physics, but a waveform collapse is just so bizarre that we can't even guess that much.

-FeyCGirl

damn i can't even wrap my head around the idea that physics would stop working.

-mrbuenopersonita

And I just realized I can't even wrap my head around the idea that physics keeps working.

-EnragedFilia

2.

Sharks are currently endangered. If they died out, fish populations would rise, which would reduce populations of algae and seaweed and other stuff that fish eat, which would in turn bring down the fish population lower than it was originally, which would cut into the populations of land animals that eat fish like bears, which would increase the populations of deer and other prey animals, which would reduce plant populations, which would reduce all animal populations, which would decrease human population.

tl;dr Don't mess with the food chain.

-Kitsune_of_the_mist

1.

Neutron star gets on a collision course with Earth. The documentary I watched on this posited that humanity could survive it with a generation ship if enough people banded together quickly enough, but its equally possible that we'll just self-destruct by the time it reaches us and tears the planet apart.

-I_Ace_English