The Universe is an infinitely hugely mindbogglingly unimaginably vastly holy shit unending big space of mostly nothing, where we as human beings are even more of nothing. And contrary to popular belief we do not matter at all in vast schemes of things as far as the Universe is concerned. To put the unfathomable endlessness of the Universe in perspective, all we have to do is to take a look at the Hubble Ultra Deep Field. We may have an advantage of being the "intelligent" species with a large number of tools at our disposal, but we still know practically nothing about the Universe or how it really functions. In fact, it is a miracle that we have survived this far, given all the supervillains who are lurking all around us in the Cosmos ready to send us packing to Kingdom Come at the first available opportunity. We know that more than 99% of species that ever lived have gone extinct as of today, and some of these species had dominated the planet for millions of years but were totally wiped out in an instant. And there is only all possibility that it will happen again, to us too. In fact, without even knowing that it happened, we can be wiped out the next minute you along with everything else in on the Earth or even in the Universe could be dead before you finish reading this! Not to panic, it might not happen before you finish reading it. So go ahead! Enjoy!
5. Geological Hypercatastrophes (Super Volcanoes, Earthquakes etc)
We know all about this thanks to Roland Emmerich. Natural mega catastrophes can hit us any time, anywhere and without warning. And the Earth has blown up regularly with a vengeance during its history at regular intervals whenever Nature wanted to let off some steam. This is particularly awesome because one event will set off a chain of others, usually aided by helping hands from Cosmic Impacts. The recent fate that Japan endured shows us how even the most developed nation on Earth will become helpless when faced with a disaster of huge magnitudes. With all our scientific advances, we still have to rely on rats and horses to know when the next Earthquake strikes! And an earthquake powerful enough can set off other events that can cause mass destruction like flash floods and mud slides due to dam breaches and volcanic eruptions!
The case of the Yellowstone caldera is relatively famous, but what is often overlooked is that the entire frigging park, all 3960 square kilometers of it is a Volcano! And this volcano which explodes approximately every 600,000 years. And the last eruption was, guess it, 640,000 years ago! We are overdue. There is a detailed explanation available on what would happen if it blew tomorrow, and it is exactly what happened in the movie 2012. Another example, the Toba Volcanic Catastrophe, which happened 71,000 years ago and blew up much of present day Indonesia, caused ash deposits of 15 centimeters throughout India covering everything, blocked sunlight for months, killed off thousands of species altogether and is believed to have brought proto-humanity to the brink of extinction with only 1000 mating pairs remaining! We have worked our way up since to 7 billion with some dedicated mating ever since. We might not be so lucky the next time.
It lately happened: Happens all the time.
Scale of extinction: 70%
4. Exploding Megastars (Cosmic and Gamma Radiation)
Supergiants and Hypergiants are the biggest, unimaginable huge stars in the Universe, thousands of times larger than our Sun. They die the most violent and spectacular deaths as Supernovas and Hypernovas, exploding with such intensity that the resulting light shows outshine entire galaxies for weeks. Looking good as it is, this is extremely bad news for us. These explosions will generate insane amounts of Galactic radiation known as Gamma Ray bursts, which IF are directed towards the Earth will spell doom for all cellular life. We exist only because the Ozone and other protective layers guard us from harmful Cosmic radiation that exists in space from frying our sad tissues to a crisp. Gamma Ray bursts will first fry the Earth's protective layers and then along with other Cosmic radiation like UV and X-rays the Earth itself. This will cause burn deaths, wiping out of almost all plant life and so on.
Gamma Ray bursts due to star explosions are estimated to happen once a day, but most are far away enough to cause any serious damage. There are millions of Supernova candidates in the Universe with plenty of them nearby in our Galaxy itself which can fry the earth like a peach in a microwave oven in case they explode.. We have been watching those stars, but have no idea when these will explode or if even they already have and do not exist right now. Betelgeuse, the most well-known Supernova candidate is about 640 light years away from us. It might already have exploded 600 years ago for all that we know, but we will know only when the light reaches us after 40 more years. Or tomorrow. Or the next second. But thanks for small mercies, Gamma Ray bursts are emitted only from the poles of stars, and we should be safe if they do not point towards the Earth. Hopefully
It lately happened: 444 million years ago, when it caused mass extinctions of 90% of all life
Scale of extinction: 80%
3. Space Rock Impact Events (Asteroids/Comets)
The superstar among doomsday events, this is also the most popular, studied and anticipated. Impact events are the most preferred vehicle among nature's extinction strategies, and a large impact occurs every 100,000 years or so. An object 10 km across will hit the Earth with the energy equivalent to 200 billion megatons of TNT and anything within a thousand kilometers will be instantly wiped out. The sky and everything else will be set on fire by superheated dust and rock hurled into the atmosphere, which will then form hot, rolling dust clouds which will block out the sun for months ushering in a nuclear winter and dispatching most of what survived the initial carngae in a slow, agonizing, painful death. And it just doesn't end there. The impact will cause rips and holes in Earth's magnetic fields and protective layers, huge mega tsunamis, earthquakes and volcanic eruptions all over the Earth and also pole reversals, axis displacement, crust displacement... In short, every single one of nature's weapons of mass destruction will be used all at once like a hellish dance of death aimed at epic total mass annihilation.
Now, the question is, how can this happen suddenly? Aren't the NASA or some other people watching the skies for us? Well yes, but with the resources, methods and means for this we have today are woefully inadequate. Space is vast and there might be an infinite number of potential objects out there, hidden away and easily missed until it is too late. Just around hundreds are being discovered every year, actually, they discover new potential threats all the time! It wouldn't be a wonder if suddenly one day a comet pops up from behind the moon giving us a day's warning. And make no mistake, YOU will not know what hit you until after it has hit you because no one will warn you, even if they know it. And then, even Bruce Willis will not be able to save us.
It lately happened: 65 million years ago, when dinosaurs were wiped out
Scale of extinction: 90%
2. Black Holes (or Runaway Stars) Dropping in to say Hello
Now this is the real spectacular stuff. Imagine you are just walking around, doing your regular day to day stuff, when suddenly everything around you starts "unraveling", or coming apart like strands of spaghetti or Maggi noodles, before everything goes black as you are ripped apart at molecular levels and crushed to a single point with everything else. Congratulations, you just have been sucked into a real Black Hole, you know the part of space-time which acts like a ever-hungry cosmic vacuum-cleaner with gravity so infinitely strong that even light cannot escape it. And then there are runaway stars ejected from their orbits randomly wandering around at high speeds. Most of these are super massive, and can alter their course dramatically to head wherever they want to. The advantage here is that we can see these things, given their size. But no such luck with Black Holes. There might be one knocking on the door right now, waiting to yell "Surprise!!"
This one, unlike the VME, is not just theoretical, but can happen anytime as there may be millions of "rogue" black holes just wandering around the Galaxy looking for juicy planets and stars to snack on. And if one gets near enough to the solar system, it will just gobble up parts of it or at least rip the system apart. The fun part is that we cannot really know when a Black Hole approaches because simple enough, we cannot see it, and can know of it's presence only when it interacts with some other celestial body. The nearest Black hole is said to be only 1600 light years away, which in Universal scales is just in our backyard! And scientists are still confused on where it actually is, which shows how specific the science actually is. So good luck with that!
It lately happened: Heh. We wouldn't be here to talk about it.
Scale of extinction: 100%
1. Vacuum Metastability Event (The Universe just reboots)
This one is just so insanely mindf**kingly fantastic that it cannot fit in the thinking mind levels of ordinary humans. You have to be either a super-mega intelligent human or totally crazy. Or both, that helps. This (so far), is just a theory developed from deep inside called Quantum Physics. It deals with things like subatomic particles and what sub-atomic particles generally do at sub-atomic particle levels, which is best understood when high at planar and cosmic levels. The theory goes like this: "Chaotic Inflation theory suggests that the universe may be a false vacuum or a true vacuum. If a bubble of lower energy vacuum were nucleated, it would approach at nearly the speed of light and destroy the Earth instantaneously, without any forewarning."
In English, this just means that the Universe if finds a state of energy in which it can exist which is lower than the state of energy now, it will just reboot like a PC running Windows XP. That is, everything that is there now will immediately cease to exist, and another Universe will emerge with a set of different Universal constants, where none of the laws of Physics or Chemistry which are present today will apply. For example, light will travel at a different speed, Gravity will not be the dominating force and Hydrogen and Oxygen if combined will probably explode instead of forming water. Also, that chick who spurned you will fall in love with you (probably).
It lately happened: Some weird theorem suggests that the Big Bang was actually a VME. And that we are living inside a Universe which is actually a Universe within an electron of another Universe and there are infinite Universes with differing set of rules and... Screw this. Where is my bottle?
Scale of extinction: 200%
If Hollywood teaches us anything, there are quite a number of other doomsday scenarios, ranging from Alien Invasion to Solar Flares to Global Warming to Zombie Apocalypses to Nuclear War to Machine Domination to Earth's Core Failing to LHC Failure to God Himself coming down to Earth and even everyone on the Earth just going nuts. But probably none of these will ever happen. What we know is that the sun is still shining, we are all alive and breathing, we can see the trees, feel the wind and watch the rain fall, we can hope for tomorrow and live for it. So the best case scenario here would be to drop cloak of know-it-all and self righteousness, feel the sun, wind and rain on your face, smile, show a bit of kindness to your fellow beings and be happy that you are alive. It is a gift to be alive.