# Repeatable, Unavoidable Cataclysmic Events

Lets say you had a planet which has held many, many prosperous civilisations, each one technologically advanced but not space faring, what sort of cataclysmic events could wipe each one out which are unavoidable, repeatable, and leave enough survivors to evolve into the next?

I was considering meteors but they are too sporadic to count as being repeatable.

• Do they need to be periodic? Every 10,000,000 years or can they be random? – Liath Oct 8 '14 at 12:01
• I'd like to say random, but with a minimum gap of about 500 years in between so a civilisation can actually build up. So by repeatable to the point that they're inevitable, but doesn't have to happen every X years. – James Hunt Oct 8 '14 at 12:04
• Solar flares might be worth looking into, though I'm not sure how plausible they would be. – overactor Oct 8 '14 at 12:04
• It's all a deterministic computer simulation, and every now and then the user pushes the reset button when his boss looks over his shoulder. – yters Oct 8 '14 at 15:31
• Re: impact events, Rampino et al have shown somewhat persuasive evidence of periodic impacts due to perturbation of the Oort Cloud (essentially the solar system is surrounded by comets) which is possibly responsible for comet swarms. Also, it's not infeasible for just one big comet to be broken up into millions of pieces and on approach (like Comet Hale-Bop) manifest as impactors rather than shooting stars (which is dust). My point being, don't discount the effects of impact events too quickly, as the February air-burst in Russia showed us, impacts are a very real occurrence. – Aussie Cryptocurrency Oct 8 '14 at 15:49

Something astronomical is probably your best bet. Meteors are good, but like you say they're not a sure thing -- even if you get some good hits on your planet, it doesn't ensure an extinction-level event like what you want (note that "extinction-level events" don't necessarily mean that all life ceases to exist -- otherwise our own planet's biosphere wouldn't have continued through 6 previous events!).

To get a lengthy period between events, you'd need your "extinctor" object(s) to be on highly elliptical, very large orbits. We're already familiar with lots of these in the form of comets -- Haley's Comet being probably the most famous, returning every 75.3 years.

But if we have a big object that comes around and smashes into the planet, that's it -- it's not repeatable. And comets' tails just don't have the "killing power" necessary to pull off an extinction (well, they might, but they'd most likely have to expend too much of their mass in each pass to be repeatable for long).

So we're probably looking for an object that effectively "radiates" death.

We could put a particularly massive object into our highly elliptical orbit. If it's big enough, it could inflict cataclysmic tidal forces on the planet, pulling tectonic plates apart and triggering gigantic earthquakes and mass eruptions all over the surface, effectively wiping out most life and certainly devastating any civilization that might have built up.

The obvious answer here then is a Jupiter-sized -- or bigger! -- gas giant. On the other hand, passing so close to the sun may cause the latter to "suck up" too much of its atmosphere, and we again have the "crumbling comet" problem. So a stony giant planet may be a better bet.

Now, people are going to see this thing coming from a long way away -- we can already see Jupiter with the naked eye, and it's so far away our own tiny little moon has more influence on our tides! Maybe that's what you want, but I've got something else in mind.

It's one thing to see a big dot in the sky growing bigger and bigger until it passes so close that it destroys you. It's quite another to see something you can't even describe, let alone understand, do the same thing.

Can you imagine what you'd think if you saw the stars in your sky doing this:

You'd see this bizarre, mysterious, and terrifying effect moving ever closer and closer, affecting more and more of the sky, until eventually your world is destroyed amid earthquakes and volcanic eruptions. One heck of a terrifying harbinger of the apocalypse, eh?

You might think this is a black hole, and in fact you would be right. Just don't get caught up in the popular hype about black holes "sucking in" everything in sight -- outside their event horizon, they're basically no different than any other massive body. They just happen to be so dense that their surface (if they even have one) is lower than the point where their gravity is so intense that light itself can't escape -- but it can and does bend around it, creating the "lensing" you see above!

To make it more mysterious, you could call it a Dark Star instead. They were a legit theory under Newtonian physics, though apparently Relativity means they've basically been replaced by black holes as the consequence of a super-dense object; if your society goes extinct before they discover Relativity and/or Quantum Mechanics, but after Newtonian Mechanics, they would likely theorize a dark star rather than a black hole. Plus, it's a scarier name than the over-done "black hole" (just my (not-so-)humble opinion).

You can put this thing on just about any orbit you want to achieve your apocalypse at any interval. If it isn't in sync with your planet's orbit, you'll have variable degrees of the apocalypse on each pass, growing more and more devastating until it peaks and then becomes less and less devastating -- but always plenty to wipe out civilization and leave only scattered survivors on an almost unrecognizably changed landscape.

And for bonus points, your civilization could reach the early space-faring stage, and so long as they can't escape the solar system to colonize another there's nothing they can do to save themselves -- after all, this is literally a second sun coming to wipe them out!

• Whoa - upvote for the gif and comparison between a black hole and a dark star! – KaguraRap Oct 8 '14 at 19:17
• Kudos go to @BartekChom for updating and correcting that comparison. I'm not really convinced that Einstein's relativity invalidates dark stars, but I'm letting it stand because he's already shown he knows more than I do on this topic -- so he could very well be correct. (Plus, Wikipedia seemed pretty adamant about making it a point that dark stars are a consequence in Newtonian physics, so...) – Kromey Oct 8 '14 at 19:24
• A bonus would be that information about this event can survive onto the next civilization in the form of legends. And this next civilization will regard it as just pure myth, until it starts becoming detectable again. This could start all kinds of doomsday cults many years or even centuries before it starts getting dangerous again. – vsz Oct 8 '14 at 21:09
• How would this object destroy civilization without destroying the planet itself? – hairboat Oct 8 '14 at 23:03
• @abbyhairboat That part of the theory should be fine. It can come close enough to cause massive weather distortion, hundred meter tides, earthquakes, volcanoes, etc ... but not close enough to rip the planet apart. A bigger problem would be the (fairly high) likelihood of it pulling the planet out of orbit. – Tim B Oct 9 '14 at 6:51

Meteors

Actually meteors are a possibility here. If you had a swarm of meteors (for example a shattered planet) that traveled together and orbits intersected at regular intervals then there would be a chance each intersect that the meteors would strike. Sometimes you would get lucky and none hit, sometimes not so lucky...so you can explain intervals with no strike.

Rather than having a cluster you could have a gas giant with an eccentric orbit, whenever it intersects with the asteroid belt or oort cloud or similar it sends a swarm of rocks heading towards the sun. Again these may or may not impact your planet but would happen on a regular basis.

Solar Activity

Maybe there is some unknown mechanism happening within the sun that causes cycles of activity. Every X hundred years these mechanisms cause the sun to release a massive solar flare that wipes out everything on the sunwards-facing side of the planet and lasts long enough to get a chunk of the night side as the planet rotates.

Tectonics

There could be a similar cycle happening within the planet itself. Pressure builds and then at regular intervals erupts in cataclysmic volcanic activity.

Life

There could be something similar to our blue-green algae that blooms and is poisonous but turned up to 11. When the climate, conditions, food and everything else is just right it blooms and blooms, poisoning the entire ocean and then the air and wiping out most animal life.

Snowball Ice Age

A full on snowball ice age again could happen on a repeatable sequence depending on the factors that trigger it. This would squeeze life down to a smaller and smaller area at the equator, potentially even covering the entire planet in ice. This would tend to have cycles of at least thousands and possibly millions of years though so may be too slow your uses.

• The Snowball Ice Age could be combined with the Solar activity, making a great freeze be followed by a great thaw, essentially resetting the planet. Would just need to find a way to justify it. – James Hunt Oct 8 '14 at 12:22
• Lets say the sun in this system is variable - it stays stable for 500 years then output drops noticeably for 100 years, causing the planet to freeze over, then increases for 100 years melting it, then goes stable again. – Tim B Oct 8 '14 at 12:25
• Just a little nit-pick: ...you could have a gas giant with an eccentric orbit... In orbital mechanics eccentricity is the measure of the "roundness" of an orbit. All orbits are eccentric. The word has a double meaning in this context, which may be slightly odd when presented in the written word. (Source: braeunig.us/space/orbmech.htm#conic) Just fyi. – 0112 Oct 9 '14 at 18:46
• @alex0112 Actually I did mean eccentric orbit in both senses - if the orbit is eccentric enough then that would make it cross paths with a lot of other orbital bodies and fling them out of their current orbits. i.e. a Gas Giant in an elliptical orbit. – Tim B Oct 9 '14 at 22:05

## A Ginormous Volcano

The best candidate I can think of is a volcano at 8 on the VEI scale, induced by tidal motions of a parent planet or another passing by every x number of years due to orbit overlap.

I don't really know if tidal motions are enough for this, but it could be combined with an eclipse or something to make it more dramatic and possibly aggravate the results.

I don't know enough to number-crunch this but I think it's doable, if you massage the planet sizes.

## Perpetual night

Another possibility would be to force a perpetual night, for a span of centuries possibly due to a peculiarity in orbits, causing another planet to hide yours behind its shadow for enough to induce a spontaneous and intense ice age. This may be possible if two planets have elliptical orbits, but one is offset, thus allowing their apoapsis to synchronize every x number of years for some time.

• I like the idea of perpetual night, the end of light causing the end of enlightenment, how poetic! – James Hunt Oct 8 '14 at 12:19
• @JamesHunt - There's several sci-fi stories, most notably Asimov's Nightfall which explore the night -> civilization collapse idea. You might be interested in taking a look. – Bobson Oct 8 '14 at 14:22
• @Bobson I also immediately thought about Nightfall when I saw the title of this question. – o0'. Oct 8 '14 at 23:00

If you want something regular, you are probably looking at an astronomical event. One thing that you might consider is a binary solar system where the planet is in regular orbit around it's primary star, but the overall temperature is dependent on the secondary star around which the primary star orbits very slowly. Then either the planet freezes up when it gets too far away or heats up when it gets too close.

Anything routine should be predictable to a civilisation but without the means to get off the planet it may not be preventable and the resources they have remaining may be required just to do enough to preserve the species through the next pass.

You might look at Brian Aldiss Helliconia books for an interesting take on this. The setting of Anne McCaffrey's dragon stories is also afflicted by a routine but dangerous ( not apocalyptic in this case ) astronomical effect.

• This would explain the many year long seasons in Game of Thrones, too. – hairboat Oct 9 '14 at 12:21
• Indeed- I don't know if it is mentioned in the books, but it certainly seemed to be clearly implicit. – glenatron Oct 9 '14 at 12:46
• I would do it the other way around: Have the planet orbit the larger star and place the smaller star on a highly elliptical orbit. Every n-thousand years it enters the inner solar-system and drastically increases the temperature of the inner planets while it gets close to their orbits. – Philipp Oct 9 '14 at 13:29

The videogame trilogy Mass Effect has a really similar situation: Every 50,000 years, the current galactic society vanishes, leaving behind a galaxy-wide network of hyperspace routes, as well as a central hub where the Galactic leadership stations their leaders. A few thousand years later, the next spacefaring species finds this network and starts activating it.

Halfway through the first game, it is revealed that every 50,000 years, a huge armada of extragalactic invaders called "The Reapers" invade the Galaxy and harvest all space-capable species, leaving behind the ruins of their society for the next civilization to find. They are also the creators of the hyperspace network and the central hub, to ensure that galactic society evolves along a chosen path. There is logic behind this: Apparently, the Reaper invasion is a way to prevent artificial life from rebelling against the creators, although the method they use is somewhat counterproductive.

• Upvote for Mass Effect reference. XD And for the superior species or other-worldly forces managing the lesser species for cycles. – KaguraRap Oct 8 '14 at 19:25

There was a grest sci-fi story about a civilization (A planet with 3 suns I believe) where some researchers were just realizing that their civilization had been wiped out every x years.

Turned out that at least one of the suns was always visible except for this short period every x years, and because of that they never had invented any light sources which caused them to burn everything... but the real problem was stars--the sheer number of which drove everyone insane.

• I believe you are referring to Nightfall by Isaac Asimov. – bowlturner Oct 8 '14 at 19:21
• I believe in Nightfall, it was two binaries orbiting each other, and two other suns orbiting the two binaries together. Making it 6 suns in total. :) – Bjarke Freund-Hansen Oct 9 '14 at 10:33
• I think nightfall is correct, I don't think I've read it since the 70's so sorry if I got the details wrong :) – Bill K May 2 '18 at 22:55

I'm writing this here because I want to propose a solution to my own question based off of the previous answers (Tim B and ivy_lynx).

What if the original civilisation was HEAVILY technologically advanced, to the point they started construction of a Dyson Sphere (an eventuality for an advanced race), but halfway through construction it malfunctioned and stopped producing power, leaving the race on the dark side of the sun with no power, heat or light, killing them all off.

Since then, civilisations have been built up over the ruins to the point where the section of sphere eclipses the sun, plunging the planet into a period of darkness and perilous cold, killing most of the civilisation off. Then during the end period of the darkness, the sphere flickers, jump starting the original tech under the planet to thaw out the ice enough that the sun can finish the job.

Assuming the planet and the sphere section rotate the sun at ROUGHLY the same speed, they will only intersect at specific periods through time and, as it will spend most of its time out of sight, the civilisations won't pick it up.

Lacks the finesse of the natural options suggested, but I thought it was worth adding to the pile.

• Nice, I like it. You don't even need the sphere to come to life, just have the planet come out of the shadow and warm up naturally. – Tim B Oct 8 '14 at 13:00
• This a nice way to have them cause the extinction themselves and make it periodic as well (possibly having the survivors forget what happened and think that the sphere segment is natural or made by someone other than their ancestors). One thing is unclear to me though - why would the sphere malfunctioning cause any problems? If they're on a planet, the sphere is useless, if they're on the inside of the sphere, it can't eclipse the sun for them. It would make more sense to me that something prevents them from finishing it in time, while they're still inhabiting the planet. – mechalynx Oct 8 '14 at 15:14
• A civilisation capable of creating a Dyson sphere, would have to be quite advanced and space faring - at the very least interstellar. It would be difficult to envision such a civilisation disappearing due to a failed dyson sphere. – Kami Oct 9 '14 at 14:46
• The period you'd expect your orbit to intersect a Dyson Sphere fragment is less than a planetary year, and the duration would be short as well. So either the adjustment is easy...just darkness during August every year, or it is impossible and everyone dies. – Oldcat Nov 14 '14 at 23:53

It could well be self-inflicted. The Moties series by Larry Niven and Jerry Pournelle covers exactly this scenario:

The Moties breed rapidly and compulsively - non-pregnancy is eventually fatal. Overpopulation eventually leads to warfare and the destruction of advanced civilization. To minimize the risk of such wars, they long ago moved all the system's asteroids into orbits well away from the home world, so they can't be used as missiles (they have some space technology, but not enough to reach other planets). They have exhausted the planet's supply of fissionables, so there's no more nuclear war, but the craters from the old days are clearly visible from space. Because this has happened so many times, the Moties have provisions to help civilization return after each cataclysm: great barn-like archives, with locks that require some knowledge of astronomy to open. These contain samples of the technology needed to rebuild.

This may give you some useful ideas while exploring your own.

Nuclear war.

I once read a short story (Asimov's I think, but not sure) which argues that all-out nuclear war could be the "natural" outcome of technological advancement in large-ape civilizations.

Our Earth was an exception only because nukes happened to be invented at the very end of an intense and prolongued world war. By the time appettites for war were large again, the US and USSR had already achieved a fairly obvious, short-term Mutual Assured Destruction, which is why they did not attack each other.

Even if you don't agree that human nature leads to that, it could be the case for another species, a little more violent or a little more competitive...

• The Gentle Vultures by Asimov. – Oldcat Nov 14 '14 at 23:51

You could go with one of the theories for Earth's own mass extinction, which is cosmic radiation.

My understanding is that because our entire solar system wobbles a little as it travels around the milky way, every 62 million years we are exposed to enormous amounts of radiation when the earth peaks up enough above the plane of the galaxy.

National geographic has an article about it.

• There's something wrong with their math. A 225 million year cycle reaches tilt extrema every half cycle, or 126 million year, not every quarter cycle. – Oldcat Nov 15 '14 at 0:00

You might consider a system similar to Dragonriders of Pern, where a nearby astronomical body regularly introduces some agent into your planet.

What wipes them out every time is not an external force, but themselves. Let me explain.

Every time they get wiped out, the previous civilization becomes a vague legend until it reaches the level of mythology. The facts about how it happened get lost. Over time, as the civilization advances they find more clues about previous civilization on the same world. With even more research they realize that they are descendants of those beings... and that it's happened multiple times.

Armed with the knowledge that it's happened many times and that it could happen again, they come up with theories as to what happened so that they can protect themselves and avoid it happening again. The problem is, that what kills them is those very plans they put in place to protect themselves... The plans go awry and everyone dies. The latest civilization, of course, doesn't realize this until it's too late. Maybe the clues they leave the next civilizations will help them avoid the same fate.

This leaves open the question of how it happened the first time. Sort of the opposite of a religious creation story.

• Hey welcome to the site! Nice and answer, and good thoughts. Unconventional for sure (I mean that as a complement). – DonyorM Oct 9 '14 at 5:39
• Just a note for cultural advancement :P The opposite of religious creation myths is called Eschatology. – Nigralbus Oct 9 '14 at 8:03

http://en.wikipedia.org/wiki/Geomagnetic_reversal

During a Geomagnetic reversal, solar winds would be devastating for life on earth, i guess.

• Welcome to Worldbuilding, this is what is called a link only answer. It it best practice to put the relevant parts into your answer, a nice summary if nothing else. – bowlturner Oct 8 '14 at 15:03
• A good explanation of this and how it would affect Earth would get an upvote from me. I can't upvote a link though. – trichoplax Oct 8 '14 at 18:54

Stargate Atlantis has some similar themes to the Mass Effect answer. In the show, the Pegasus Galaxy has been under Wraith control for millennia, harvesting the human population as food at (presumably) random intervals. This ranges from small-scale (handful of people taken) to the culling of large parts of the population, leaving enough to repopulate the planet for the next harvest.

More notably, they also often cull or outright destroy civilizations approaching a level of scientific and technological proficiency to become a threat to them. I seem to recall episodes dealing with a civilization where secret research has been going on for generations to stop the Wraith, having kept its knowledge written down in hidden archives to allow future generations to continue their work, even if everyone involved is taken.

Ice ages.

For the last 4 million years, the earth has spent 90% of the time locked in ice ages each roughly 140,000 years long punctuated by brief warming periods of 6,000-15,000 years. (IRRC, we're statistically overdue now.)

The ice ages may begin suddenly from having seasons to permanent snow all the way down to north Texas in as little as 20 years. Even where it doesn't snow, the climate will alter radically, the seas will recede stranding every port town in land Increased temperature differentials will drive huge and violent storms from the equators to the poles. Deserts will expand.

The effective size of the biosphere contracts by as much as 50%. People will have to migrate to survive and mega wars of desperation will be common. It's unlikely industrial civilization would survive.

1/3 of the planet covered by ice with another 1/3 turned to desert. Human survivors of the ice and the wars would be forced in little enclaves scattered all over the equatorial regions. Violent storms and changing coast lines make sea travel impossible. Trade and communications break down.

The kilometer thick glaciers will rake everything down to the bedrock destroying all evidence of man made structures. When the next ice comes, New York will be raked off and dumped in the sea. 150,000 years later, there will absolutely nothing left to provide evidence that the great city ever existed.

Even the end of the ice ages could prove as devastating to any civilizations that did rebuild. Massive floods would sweep out from the melting glaciers. The sea levels would rise, perhaps in as little as 20 years again.Since most great human cities in history have been built by the ocean or in flood plains, it's likely few would survive. Most of human works would be flooded under the sea with only the tropical mountain inlands providing refuge.

So, every warming period civilization would have zero knowledge of the previous warming period civilization and likely not even the civilization of the ice ages themselves. They might find bits and pieces here and there but nothing useful or cohesive.

If you set the story on a world with a different continent pattern than earth so that more of the land mass was towards the poles, then the effects of an ice age would be even more severe.

This is building on the polar jets answer of Lorren Pechtel.

My scenario involves an unknown mechanism that produces gamma ray bursts at fixed intervals. For the purposes of world building, lets say that instead of a black hole, it is a neutron star and sun-sized star pair that over the course of 20,000 years erupts in a gamma ray burst from the neutron star reaching a critical mass after siphoning enough matter from the sun-sized star. (This is not scientific, but for the purposes of world building we will be taking liberties with physics).

To add in just enough science to make this science-fiction, the gamma ray burst would have the effects described in the wikipedia page on gamma ray bursts - it would deplete the ozone layer and cause mass extinctions. This would require us to place our solar system at a distance of around 10,000 light years from the bursting neutron star. This would be unavoidable because since the event happens every 20,000 years, there would be no chance for a civilization to go to that star and stop it in time if they go at some fraction of the speed of light, assuming they could even discover a mechanism to stop it.

Not only would the ozone layer be depleted, but the side of the planet facing the gamma ray burst would experience lethal doses of radiation. On the other side of the planet, the people in power would have enough time to retreat deep into underground bunkers. These bunkers would have been created in preparation for possible nuclear war with rivals on the same planet, since the discovery of nuclear energy would invariably lead to the possibility of nuclear war for each civilization cycle. The bunkers would also have enough food to allow the survivors to stay in for centuries before they emerge onto the world.

The survivors would spread throughout the world, memory of the bunkers and the gamma ray burst will fade, the bunkers would fall into disrepair and be lost to time, and the next civilization will have no idea what happened, save for the odd technological relic discovered here and there and layering in soil created by the extinction events.

In the foreword to John Brunner's The Crucible of Time, the author says something along the lines of:

"It has been speculated that in the history of our own planet, passing through bands of interstellar dust led to ice ages and catastrophes. I wanted to explore what might happen on a planet where such events happened at the same time as life had just started evolving towards intelligence".

There is a black hole in the system. Periodically one of the polar jets fries the world.

• This is too short for an answer - either elaborate or post it as a comment to the question. – mechalynx Oct 9 '14 at 1:02
• Why should I add words when one line communicates everything needed? Just because it could fit in a comment doesn't make it not an answer. – Loren Pechtel Oct 9 '14 at 3:15
• Not everyone knows how black holes work, especially when it comes to their jets and especially on a Q&A that doesn't specialize in a technical field. Some numbers and explanations on what the actual catastrophe would be like, it's extent, why it would repeat periodically or how to work it into the world, would all be important aspects to your answer. What seems obvious to you may not be as clear to someone else, especially when dealing with technicals. If you're just relaying an idea, a comment should suffice - an answer is when you elaborate enough that it becomes practical to use. – mechalynx Oct 9 '14 at 3:22
• @ivy_lynx I don't think anyone knows just how powerful the jets will be, and the matter around make a big difference also--it's not an even remotely fixed value. I would let the author decide how much damage it does. – Loren Pechtel Oct 9 '14 at 3:26
• These things can be calculated - but even if you don't want to provide exact numbers because it's too much work and there's too many factors involved, you can provide a link to an article that states a magnitude, repeat the relevant magnitude here and give an idea of what the damage would be (would the oceans evaporate, for example? would buildings survive?). Also, it would help if you give an idea of how the repetition would happen - why does it destroy the surface occasionally but not all the time? – mechalynx Oct 9 '14 at 3:30