There was an event in the 1800s caused by solar activity where telegraphs operated on their own without power and I think caused minor damage. Should such a solar event happen again, it would destroy all of our internet network capabilities and other electrical gear. 'The Carrington Event.' -123eyecansee

If you get diagnosed with Alzheimer’s at 65, you had the disease since your early 40s at least. And you experienced very mild symptoms but didn’t notice them. And your brain fought like hell to compensate for the deficit. When you get diagnosed, your brain is already very severely damaged and will never recover from the deficit. -Alkuna

Our white blood cells are fighting cancer cells on the daily. -bataract93

The fact that we can just get a blood clot and die and not realize anything was up, the human body has so many ways of just suddenly dying and it's terrifying. -a-packet-of-noodles

The metaphysical aspect of consciousness. Where is consciousness? What is it? When does it really end? Where does it go when we die? When we sleep? How does it occupy our entire being without being physically present? We are quite literally the universe experiencing itself, in a fragile little bubble on a mote of dust in a sunbeam.

And yet, what exactly are we? That’s the sort of question that keeps me up at night. I don’t fear death, I don’t fear world annihilation, but I am deeply unsettled by the mere experience of being aware. -Bloorajah

When the atom bomb was being created, the leading scientists associated with the project at the time had to calculate the flammability of the Earth's atmosphere in order to ensure that detonating the bomb would not cause the atmosphere to combust.

At the time when the first atom bomb was detonated, these scientists still had not answered this question, meaning that we legitimately just crossed our fingers and hoped we wouldn't set the fucking planet on fire. Humans are stupid. -Otherwise-Win7936

There is a form of insomnia that forces your body to never be able to sleep, and has a 100% fatality rate. -thealientomatoes

The Dark Era of the universe. When all the star fuel is gone (and it will be) and all the white dwarves have gone cold and dark (and they will) and all the black holes have evaporated away into elementary particles (and they will), the universe will be a cold, dark place... Forever. -Imissyourgirlfriend2

Rabies. You can have it and never know until it decides it's time. And then you'll die terribly. -PMyourTastefulNudes

Space just in general. Or that bit about the higgs field not being in true equilibrium and that returning to it would break everything as we know it. -PM-ME_UR_TINY-TITS

In wastewater treatment plants they aerate the water so the bacteria breaking down the poo has plenty of oxygen. Due to the introduced air, the water density is low enough that a human body (or most any object that would normally float) will go straight to the bottom. -MustNotSay

The Marianas Trench is so deep that you could fit Mount Everest in it. Even if you stand at the summit, there would still be about a mile of ocean above your head. -Tziporah-296

That one day I will die. I don't know why, but sometimes at night, as I'm falling asleep, I suddenly think of this and it freaks me out. Like start feeling super anxiety. -nastybacon

The Pacific Northwest has a 600 mile long subduction zone that is approximately 70 years overdue for a 8.8 to 9.2 magnitude earthquake, and nothing on that coast is built to withstand that. There has never been a strong earthquake on that coastline since European contact, because the last major earthquake happened in 1700.

Unlike Japan, which had a comparable earthquake in 2011, the PNW isn’t ready for it and things aren’t built to withstand what’s coming. Some day soon, Seattle, Vancouver, Portland, and Victoria are all going to lurch 6 feet inland, shake violently for 5 to 7 minutes, and then get hit with a 100 foot tall tsunami.

It is projected to kill tens of thousands, displace millions, and wipe out trillions of dollars of wealth; the 2nd largest humanitarian disaster in North American history, second only to Haiti. And it seems like people are just pretending this doesn’t exist. -pineappledan

That viruses cannot classify as a living organism and yet behave as if they are fully aware of their surroundings and evolve to suit it. They behave like they have a brain but are merely nucleic acids. -Belachick