At some point in the past, the once grand (or seemingly grand) world was destroyed, and it was changed into what it is now. However, what exactly took out the old world isn't always known to the viewers (or even the characters!); it's either never discussed at all, or at best hinted out with a few details a sharp-eyed viewer or reader may see or read and be able to piece together. In a long series, it may be explained in bits and pieces (or, if the audience is lucky, completely) but, generally, there will still be many gaps.
Basically, we're talking about a very dark and extreme type of Noodle Incident.
Why this is done can vary. In some cases, this trope can help bring a sort of connection to the characters; After the End, information and knowledge is going to be lost or piecemeal at best, and when in a collapsed society death is ever-present, people are going to be more worried about day-to-day survival than times past. In other cases, the creators employ this trope since the cause of the fall simply isn't important, but rather, the world that comes after is the main focus.
However, as the series goes on, this trope can end up taking a backseat if the creators decide to expand a bit on the end, having this trope only count in the earlier parts of the work, and not in the later parts. This trope can also be downplayed; why the world ended may be known, but what caused the why may never be explained. For example, the audience may know that zombies or a nuclear war destroyed everything, but may not know why zombies came about in the first place, or why the war was even fought. And if they do find out, any information they'll get would be piecemeal.
- Humanity Has Declined: The human race is slowly dying out; exactly why is never spelled out, and the humans seem no worse than melancholy about the notion.
- Sound of the Sky: 300 years ago, there was a great war that left humanity in shambles, but otherwise nobody really knows what happened. Most of the environment is uninhabitable, with the remaining population just trying to live in what few regions are still fertile.
- Sunday Without God: The legends say that God abandoned the world fifteen years prior, and as a result no one can truly die or give birth. Other characters doubt this story, believing instead that one day, everyone became a Reality Warper and collectively wished to live forever, thus distorting the nature of death. Whatever the reason for why the world is slowly coming to a standstill, the characters have to deal with the lack of true death and the consequences of people's deepest wishes coming true.
- In Y: The Last Man, it's never definitively revealed what caused the Gendercide that kills every male mammal on the planet (with a handful of exceptions). There are multiple theories purported, some clearly bunk, others somewhat plausible, none entirely satisfying. Word of God has it that one of them is correct, but refuses to say which.
- The Walking Dead. A smaller example. We know that the dead walking again caused the end, but why it started happening is never really discussed. Word of God has even said that he has no plans to explore the reasons either.
- The details involving the nuclear war in the 1970s Planet of the Apes series are never explored. All that matters is that the apes inherited the world.
- The film Stake Land involves a vampire apocalypse devastating North America (and the rest of the world including the Middle East, except apparently for the northern-most parts of Canada) and two men heading towards this "New Eden". There is never any specific information about where the vampires came from or how they came to exist.
- What caused the end in Logan's Run is never mentioned, in contrast to the novel, where overpopulation is stated to the cause for the dystopian society.
- Children of Men: It is never revealed what caused the Sterility Plague that resulted in the Childless Dystopia and no-one seems to know what caused it in-universe as well. There is also quite of few mentions of Noodle Incidents in New York and Madrid that suggest that the rest of the world is being turned into nuclear wastelands or lawless war zones, especially if you believe the government propaganda.
- 9: It is heavily implied some sort of war in an Alternate Earth wiped out humanity, leaving our creations to inherit the Earth.
- The Book of Eli has Denzel Washington in a world decimated for an unknown reason (heavily implied to be nuclear warfare). A number of people who were alive at the time of "The Flash" still suffer burns, scars, and blindness from the event.
- Pandorum starts with a Sleeper Ship called the Elysium travelling towards a distant planet for colonisation when they receive a message from Earth telling that they are all that remains of the human race. Following flashbacks mention that the Elysium tried to contact and then find Earth, but it had been completely vaporised somehow, and no theory is of help at the current moment in the story.
- There are multiple reasons given for the Zombie Apocalypse in Shaun of the Dead, including a new super-flu, GM food, radiation from a downed satellite in reference to Night of the Living Dead (1968) and it is intentionally vague to which one is the true cause.
- While it's not completely unspecified (World War III erupting), there is one detail that is very specific, which various characters ask throughout the film for the sake of trying to feel better, and that it's deemed totally unimportant to the rest who have lived to see The Day After: who fired the nukes first, the Russians or the Americans? The reason within the film why the rest don't care is because the world is devastated either way and knowing this just won't fix anything. The reason in Real Life why this goes unmentioned is because the producers wanted to showcase the horrors of nuclear warfare and having the plot pointing fingers would be counter-productive—and interestingly enough, this is the reason why the film was not Backed by the Pentagon.
- Perdido Street Station series. The khepri of the Bas-Lag novels fled their native continent en masse to escape a disaster known as "the Ravening". Whatever it was, it traumatized their kind so completely that none of the survivors of their 25-year sea journey to Bas-Lag ever passed on the details of the catastrophe, or of the ancient khepri culture it destroyed, to their descendants.
- The Road. What brought upon the end of the world is never really stated; theories ranging from asteroid impact to a nuclear war have been brought to the front. Whatever it was, it is of little consequence to the characters, who will be dead anyways at some point as the biosphere is now dead.
- The Giver Quartet. Gathering Blue mentions The Ruin, a recurring event throughout history that has brought humanity to its knees again and again. The last time it happened, it is implied to have been to modern civilization. While Gathering's, Messenger and the village that takes in Claire in Son have regressed to medieval level tech (with a few smackings of Schizo Tech and Lost Technology here and there) The Giver (and a few briefly mentioned, but never explored societies) managed to end up with futuristic technology. There is an unspecified society that owns the boats that deliver supplies to the Community that is at least at industrial level, that apparently didn't dip into Sameness, but the series never goes into details about it.
- The Hunger Games. The event that caused the fall of this world and the rise of Panem is never stated. It is hinted to be some sort of environmental catastrophe though. What ever it was, for whatever reason, one consequence is that planes have to fly low to the ground, since the upper atmosphere is apparently non-existent.
- Ford Prefect from The Hitchhiker's Guide to the Galaxy is the son of the only survivor of the Collapsing Hrung Disaster on Betelgeuse VII. His father was never able to satisfactorily explain what a "hrung" is, why it had collapsed (or why it chose that planet to do it on), or how he's survived it himself.
- In Sime Gen the fall of "our" civilization was caused by the mutation that split humanity into the titular Simes and Gens, but what caused the mutation to occur in the first place has no definitive explanation and, according to the authors, will never have a definitive canon explanation, though in character speculation is fair game.
- Malevil has the characters trying to survive in the post-World War III, nuclear warfare-inflicted chaos that assails the French countryside. At no point it is ever found out what triggered the nuclear exchange-whether it was a type of Failsafe Failure, flat-out war or insane General Ripper-types on both sides pushing the button, the characters speculate a lot but finally agree that this a question that will probably never be answered and holds no importance in light of their current plight.
- The Day of the Triffids, to some extent. The mysterious green flashes that drive nearly everyone in the world blind are alleged to be caused by a malfunctioning Kill Sat, but this is explicitly nothing more than an educated guess on the protagonist's part.
- The origin of the Triffids themselves is equally unknown, they simply started growing all over the Earth at the same time. The theory put forward in the book is that they were engineered by the Soviets, then spread when a plane attempting to smuggle samples to the USA disintegrated at altitude allowing the wind to spread seeds everywhere. However, this is based on the claims of a single, now dead, person and is again presented as nothing more than a best guess by the narrator.
- Cory Doctorow's short story (eventually turned into a comic) "When Sysadmins Ruled the Earth" describes an unspecified apocalypse from the viewpoint of a system administrator, who has to go to work late at night due to a server issue. While there, he gets a call from his wife, who tells him that their infant child has already died from an unspecified cause, and she herself dies while on the phone. Very quickly, the sysadmins lose contact with the rest of the world. Eventually, they manage to re-establish contact with others like them via the still-working Internet and establish an Internet-based government. Word of God is that this trope is invoked deliberately, since a regular person is highly unlikely to know the details of a sudden apocalypse and likely wouldn't care anyway. They get an email - purportedly forwarded from Health Canada - that the whole city’s quarantined because of a biological weapon, but as everything falls apart very quickly, it's never confirmed.
- Enforced in the Life After People film and series, as it's explicitly stated to be the story of what's left behind after humanity vanishes, not why/how we disappear.
- The Flip Side Of Dominick Hide had some sort of apocalypse at the end of the 20th century which left most of the planet contaminated with pollution and poison, but it isn't specific as to what did it.
- In Firefly vague allusions are made to Earth That Was, implying that something cataclysmic happened to cause an exodus. Details were never given, so it could be anything, from destruction to simple overpopulation.
- A recurring That Mitchell and Webb Look sketch later in the run known as "The Quiz Broadcast" takes place after an apocalypse referred to as "The Event" (remain indoors). Exactly what the Event was in unclear, but mention is made of extreme levels of radiation, and all the children dying in the aftermath. Not to mention Them, an unidentified infectious disease, and the complete and total loss of near-all human knowledge.
Host: Multiple choice: Which of Shakesphere's three plays is now thought to be prothetic of the Event? Is it A) Paracleese, B) Symbolene, or C) Boeing-Boeing?
Sheila: (buzzes in) Is it a trick question?
Host: It is.
- Gamma World from version to version, the events that destroyed the world are different, but for most versions the details remain in the dark.
- Paranoia takes place after something only referred to as the Big Whoops. But, whatever happened, Friend Computer is sure it was caused by evil Communists. Some of the Ultraviolet sections reveal that an asteroid the size of Sheboygan made its way to Earth, causing the Big Whoops. A Russian missile silo mistook the asteroid for a nuclear attack, and The Computer mistook that counter-nuclear attack as an attack by Communists (its information records were damaged, and it could only retrieve 1950's cold war propaganda at the time). They then tell the Gamemaster to use this explanation, come up with another one, or keep changing the backstory, all to keep the players experiencing the setting's trademark Fear and Ignorance.
- Rifts is set roughly 300 years after an event called "The Great Cataclysm" or "The Coming of the Rifts" nearly destroyed humanity, and reshaped the world. What's mainly known to people living on Rifts Earth is that political tensions were rising at the time, and it's assumed that some kind of war broke out, though how that lead to the Coming of the Rifts is unknown. Bits and pieces were teased out in the books for years. At present, most of the story is known. The Great Cataclysm was triggered by one South American nation nuking another South American nation on the Winter Solstice at midnight, during a rare planetary alignment. This combination of events, millions of people simultaneously dying when the Earth's magic level was higher than it had been in millennia, caused a chain reaction that nearly destroyed the world. Details are still left out, such as which nation bombed which other nation, and why. But since the cause of the Cataclysm isn't the focus of the game, we'll probably never learn all details.
- Dead Reign (by the same company that makes Rifts) takes place in a worldwide Zombie Apocalypse that was caused by a strange sickness known only as "The Wave" which killed people and made them rise as zombies. What actually caused the wave in the first place is unknown with most survivors not really caring about finding out what happened in favor of trying to survive for another day.
- Warhammer 40,000 used to have a lot of these, with human civilisation having collapsed several times in its history both before and after colonising the galaxy. Most of these have since been explained in various books and other bits of lore added over the years. However, in-universe they are still almost all mysteries, if people even know they happened at all.
- The Final Frontier mod for Civilization IV has all contact with Earth lost, and the game outright states at the beginning that it is likely that something terrible happened to it. Dialogue in tech quotes hint that Earth was trying to do some sort of transcendence project that went wrong, and destroyed the entire star system.
- While the world isn't destroyed in Civilization: Beyond Earth, it's in bad shape after something called the Great Mistake. Word of God is deliberately mum on what the Great Mistake was. All we see in images and trailers is widespread poverty and the flooded Pyramids. Some additional material indicates that natural resources are close to being used up, which is why the Seeding missions to other worlds are financed in the first place. The Purity and Supremacy victory conditions involve building a gate to Earth. The Purity-aligned faction strives to build the Exodus Gate, which has refugees from Earth pouring in, requiring them to be settled somewhere. The Supremacy-aligned faction attempts to build the Emancipation Gate (looks pretty much the same), and then sends troops to Earth to engage in some Unwilling Roboticisation.
- Metro 2033 and Metro: Last Light. What caused the war of 2013? The official website shows that the initial nuclear explosions were in the Middle East and the situation then escalated. Beyond this, all we know is that it was probably between Russia and NATO, but even that is unconfirmed. To the residents of the Metro though, figuring out the facts hardly matter.
- Enslaved: Odyssey to the West takes place After the End, in the ruins of the modern times. What exactly happened to nearly wipe out humanity is never even discussed, characters mostly focus on continuing their survival and barely question the relics of the past.
- After the End: A Crusader Kings II Mod takes place centuries after the world underwent some sort of cataclysm, with humanity having managed to work its way back up to medieval society and technology. The exact nature of The Event is deliberately left vague and up to the player's interpretation.
- In the Siege trilogy of games by Dynamix, the timeline actually starts with an unexplained apocalypse laying waste to the world and bringing humanity to the brink of extinction before bouncing back After the End. Interestingly, no one knows what happened or what the calamity even was; some speculate nuclear war, others think it was a pandemic, and at least one sect argues that the Rapture came and went and the remaining humans the leftover rejected sinners. If Dynamix's canon is to be believed, aliens were involved somehow. This apocalyptic event is simply known The Devastation. Humanity undergoes a very specified Robot War as their second apocalypse, known as The Fire.
- The Elder Scrolls:
- In a race-specific example, the series has the (believed to be) extinct Dwemer, the "Deep Elves" or "Dwarves", of all of North Tamriel, whose civilization ("Dwemereth") once spanned from Hammerfell across Skyrim to Morrowind, where the epicenter of their culture was built in and around Red Mountain. They were a highly technologically advanced race who created all manner of Magitek and Steam Punk technology, which remains unmatched by any other race in Tamriel. They were also extreme Naytheists for whom a major part of their outlook was the idea of refuting everything as real. During the mid-1st Era, they discovered the Heart of Lorkhan (the "divine center" of the dead creator god of the mortal plane) deep beneath Red Mountain. Though there are many different versions of the story regarding what happened next, the Dwemer did something with the Heart that caused their entire race to disappear from every known plane of existence in a single instant. The leading theory (which you can put together yourself in the Mages Guild quest line in Morrowind) states that they were trying to break themselves down into their base elements before ascending into divine form. However, they got something wrong with the "reforging" step and instead blinked out of existence. Other theories state that they may have even been successful in their attempt, and are currently on said "higher plane." Making it only more confusing is one particular Dwemer ruin that shows whatever happened was violent and abrupt: it's a residential area, and you find piles of ash that used to be Dwemer in beds or near their piles of equipment on guard duty. Despite the theories, no definitive answer has been given in the series to date. One single Dwemer, Yagrum Bagarn, was off exploring unsubscribed "outer realms" at the time and returned to find his people gone. He doesn't know what happened either, or why he (and apparently only he) remained, and he only lived to tell his tale because he became infected with the Corprus Disease (one of the side-effects of which turns the sufferer into The Ageless). The In-Universe mystery of the Dwemer has only deepened by the time of Skyrim since much of what was known about the Dwemer was again lost in the 200 years following the Oblivion Crisis and the eruption of Red Mountain (which wiped away a great number of Dwemeri ruins in Vvardenfell, the epicenter of Dwemer culture prior to their disappearance). Even one of the greatest experts on the Dwemer, Calcelmo, knows less about the Dwemer than some amateur archeologists in Vvardenfell during the time of Morrowind. Arniel Gane of the College of Winterhold tried to use an Dwemer dagger called Keening (the one used on the Heart of Lorkhan) and a warped soul gem to perform the ritual the Dwemer Tonal Architect Lord Kagrenac did on a much smaller scale... and he did it alright. After his quest, the Dragonborn can summon his shade from where ever he is now.
- The Great Collapse mentioned in Skyrim. In 4E 122, the Sea of Ghosts destroyed almost all of the city of Winterhold save for the Jarl's longhouse, a few surrounding houses, and the College of Winterhold (allegedly protected by magical means). Nobody knows why it happened but the citizenry believe that either the College caused it or at least could have lessened the damage while the College believe it was caused by the destruction of Vivec and eruption of the Red Mountain causing tsunamis and destroying the majority of the area. Regardless Winterhold is a shell of its former self.
- Dragon Age: Origins isn't quite After the End, but the origin of the Darkspawn who pour out in hordes to kill everyone from time to time are not truly known. The dominant religion claims they were created when a group of powerful mages broke into the Creator's city and were cursed for doing so, but the game makes it clear that no-one really knows and suggests this may just be a fable warning about the hubris of humanity.
- Dragon Age: Inquisition's main antagonist claims to be one of those mages which may support the legend being true, but it's still never shown that he's actually telling the truth, and the apparent involvement of gods from other pantheons suggests that at the least there's more to the story than that.
- A recurring apocalypse of unknown cause is central to the background of Mass Effect. Discovering the truth and then actually convincing other people of it forms a large part of the first two games.
- In Obduction, there's a theory among some of the characters that the alien seedpods that randomly abduct people to a strange planet are doing so in order to preserve the human species against some unknown but imminent cataclysm. They are correct, as seen in the bad ending (and one specific vantage point in a late-game area}: we don't know what went wrong, but it's apparent that Earth got messed up, as there are scorch marks, ruins, and massive dust clouds roaming the surface.
- All we know about the reason of the apocalypse in The Long Dark is that electricity ceased to function due to some electromagnetic storm.
- Fate/EXTRA: The world of Fate/Extra is quite different from the real world or the world as it exists in most other Nasuverse stories: something happened in the 1970's that lead to a great disaster and resulted in all magical energy in the world vanishing, never to come back. What form this disaster took, and what the immediate consequences of it were, are all unknown. All that is known is that it was a global catastrophe that altered the course of human civilization. Word of God states that the root cause was a group of vampires attempting to perform the Aylesbury Ritual to awaken the Dark Six (a kind of vampiric messiah figure) and failing terribly at it, but beyond that any details are unknown.
- Rimworld: The world you're trying to survive on once supported a civilisation advanced enough to leave behind the crumbling remains of highways and an occasional vault full of Human Popsicles, not to mention all the "compacted steel" and "compacted machinery" for your settlers to dig out of the ground and repurpose, but what precisely caused it to fall is never explained.
- Mad Max: It's revealed the social collapse and rise of gang shown in the movie wasn't the only thing. According to Max everything fall on them at the same time but he doesn't elaborate since he probably doesn't know much. All he knows is while people were bombing in unrelated wars oceans dried up and disease spread.
- In in the first game of the Danganronpa series, the last trial, the Mastermind (a.k.a. Junko Enoshima) reveals that the reason the students weren't actually trapped, but instead choose to stay inside the school for the rest of their lives was because of "The Worst, Most Despair-inducing Event in the History of Mankind" that caused the downfall of society. However, the specifics of the Tragedy aren't shown until the second game.
- Adventure Time: The Great Mushroom war is the reason behind the formation of the Land of Ooo, but for the longest time, the end was only hinted at being some sort of war. Even now, the details— who was fighting whom, the exact nature of the weapons used, how or whether it's connected to the rise of magic, etc.— are sketchy or simply not revealed.
- Atlantis The Lost Empire: Whatever exploded that caused the cataclysm that destroyed (most of) Atlantis and buried the part that was protected by the shield. All that is absolutely sure is that one Atlantean looks at another and yells out "You Fool!! You destroyed us all!" before they both perish in the tidal wave that resulted from the explosion, but what did that, it'll be forever unknown.