Eren: Salt?! L-liar!! I mean, salt?! That would be a treasure trove! Merchants would've already exhausted the supply!!
Armin: No, that's the thing! The "sea" is so huge, it can't be depleted!
Eren: That's just silly.
The world has suffered some kind of massive calamity. Humanity has survived, but at a great cost. In addition to the population decrease, much of humanity's knowledge about the world and how it works has been lost.
How can an author demonstrate the sheer scale of cultural, historical, and intellectual loss? By having the characters rediscover some scrap of information that is laughably common knowledge in the real world and react to this information with shock, disbelief, and amazement. Expect at least one character to become fascinated and obsessed with the rediscovered knowledge, vowing to travel off in search for whatever-it-is (if it's a location, animal, etc.) or to try and build it (if it's a form of technology). Expect at least one other character to scoff at the implausibility of these "facts" and question whether the long-dead writer of the text was just making stuff up.
A variant is where the characters (usually children) will hear the lost knowledge told to them by an elderly relative who remembers the days when this knowledge was common. Again, their reactions will range from rapt wide-eyed wonder to "Granny must be going senile."
Sometimes the common knowledge will be lost because that thing no longer exists in the future world (e.g.: a child asking her grandmother to tell her about that strange extinct creature called the "lion"), thus further demonstrating how far the world has regressed.
Compare Future Imperfect, where the knowledge is still there but has been misinterpreted over the years. Compare to Aliens Never Invented the Wheel, which is where aliens (however advanced) never developed something that humans consider incredibly basic. Not to be confused with Common Knowledge.
- Happens in Attack on Titan, which takes place in a world where the last remnants of humanity are holed up in a walled off territory about the size of Texas, hiding in fear of the people-eating giants that have overrun the world. Information about the outside world is not only scarce — it's forbidden by the government, and Eren mentions that the book Armin is reading from is illegal to own.
- In Gargantia on the Verdurous Planet, an ice age caused some humans to fly off into space, while others stay on Earth. The protagonist, Ledo, is from the space-faring society, but rediscovers Earth That Was purely by accident. Since the people in space have few resources and are fighting a never-ending war against another species, he's Conditioned to Accept Horror and has to learn about a society where people are viewed in more than utilitarian terms. Also, he is completely Squicked by the fact that people on Earth eat animal carcasses.
- In Evangelion 2.22, Shinji and his friends' visit to an aquarium of cloned marine life demonstrates how little they know of what the ocean was like before Second Impact rendered it nearly lifeless. Kensuke correctly identifies a sea turtle, but isn't sure of himself, while Shinji is initially confused by the briny smell because most of the ocean no longer features enough decaying plankton to create the scent.
- Invoked and Played for Laughs in Asterix and the Chieftain's Shield. At the time the volume was first published, in 1967, the exact location of Vercingetorix's capital and his last stand against Caesar, Alesia, was still uncertain.note In the comic, Asterix and Obelix travel through Arvernia, the general region where Alesia was in, and the local Arverni, still sore for the defeat, angrily (but obviously falsely) claim to not know where Alesia is and refuse to discuss it any further. A note from the authors then claims that this is the reason why the location of Alesia was eventually forgotten.
- Legends of the Dead Earth: In Wonder Woman Annual #5, the Unremembered have regressed thousands of years since their Generation Ship was struck with disaster. They have forgotten the meaning of words such as "sky," "night" and "stars" and know nothing of planets, space vessels, the universe or anything else beyond the worldship. Most notably, most male members of the Unremembered believe that women make babies "out of some contrary nature unique to them." There is a growing number of men, such as CatXon, who believe that men are just as much to blame for babies being conceived but it is widely seen as a "preposterous belief."
- Played for Laughs in The Finders, a short-lived Donald Duck series. The series is set in a post-apocalyptic world where all the common knowledge of the present has been lost, with Donald and Fethry getting the task of finding the exact meaning of many things that survived in the ruins of Duckburg. The series features a tribe of humans known as the Mistersmiths, who are trying to rebuild the society on their own based on their interpretations of what they can find (for example, they believe that dog houses are just small replicas of actual houses and that stock phrases from old cop dramas are necessary to use when arresting someone).
- Death Need Not Apply: Due to the healing effects of the Zone, children who grew up there have little knowledge of things like death, disfigurement, or permanent injury. Izuku unthinkingly asks Shinso about the strange pattern on his skin, not knowing the "pattern" is numerous burn scars.
- FFS, I Believe in You: In the sequel, a large part of the difficulty the heroes face in brewing the magic potion is that its recipe was written shortly after the events of Oracle of Ages, many thousands of years before Breath of the Wild, and thus uses names that have long since fallen out of common use and refers to things that most people have become entirely unfamiliar with. Likewise, when Link is looking through old bestiaries in Gerudo Town, most of the creatures are enemies that were very common in earlier eras but which, having since died out or migrated away, he doesn't even recognize.
- In WALL•E, when Captain McCrea (whose ancestors spent hundreds of years lounging around on the luxury space-ship ''Axiom'' after fleeing Earth) accesses the computer database, most of the facts he's getting all excited over are commonplace knowledge in the real world, like the definition of "dancing" and "farms".
- Played for laughs in Idiocracy. By the time of the dumbed-down future, people have no concept how to care for plants, and think that the sports drink Brawndo is "what plants crave." The main character tries to explain the (basic) science behind watering them, but the knowledge flies over their heads, so he settles for telling people that he can talk with plants, and that they've told him what they want is water.
- The Book of Eli:
- All copies of the Bible have been destroyed, to the point where a big driving force of the plot is various characters' attempts to protect or steal a rare surviving copy. Explained as after a terrible third world war nearly drove humanity to extinction, most of humanity decided that religion was to blame and burned religious texts en-masse. The ending reveals that Alcatraz has been converted into a massive library and printing factory, where a group of scholars work to protect and reprint the world's remaining holy texts.
- When Carnegie holds a gun on Solara outside the wreckage of the old couple's house, Eli reveals that he hid the Bible behind their TV. When Carnegie orders one of his young henchmen (born after the nuclear war) to check the TV, he replies with a blank look and a puzzled, "The what?" Carnegie rages, "You're fuckin' SHITTIN' me!" before telling Redridge (who is old enough to know what a TV is) to check it.
- Played for laughs in Sleeper. Miles Monroe wakes up in the year 2173 after spending 200 years as a Human Popsicle. The people of that time have lost most of their historical knowledge due to a nuclear war, so they ask him questions about 20th century artifacts (e.g. "chattering teeth") and people. Miles makes up a bunch of lies about them for his own amusement ("Bela Lugosi...was the mayor of New York for a while" and "Charles de Gaulle...was a very famous French chef.").
- In the world of Mad Max: Fury Road this is demonstrated when Nux, one of Immortan Joe's War Boys whose entire short lives are dedicated to fighting and dying gloriously for their master, doesn't know what a tree is and refers to it as "that thing" until someone else provides the word. In context it's quite sad.
- In The Dark Tower Roland and Jake visit some mysterious ruins from the past. Roland says no-one knows what they were. Jake: "It's a theme park."
- Not entirely lost, per se, but Lieutenant Huxley in Demolition Man is delighted to have obtained a Red Hot Chili Peppers LP (rock music is forbidden in San Angeles) and picks Detective Spartan's brain for information about 20th-century society. The rest of her squad think she's a weirdo for being obsessed with the violent old days.
- A joke from Communist Eastern Europe: Somewhen in The Future, a child reads in an old book about butter. The child asks everyone what this means, but nobody can remember. Finally, they find a definition in a dictionary: "A spread for bread, not used anymore by progressive people for its connotation with capitalism."
- Another joke is about Culture Clash between people from different times:
1: Sorry, I'm late. Long line for sausages.
2: What's a "line"?
3: What's a "sausage"?
- Some versions then follow up with a third person asking "What's "sorry"?"
- The Books of Ember:
- In The City of Ember, the simple fact that the surface world exists becomes this, as does the existence of animals, candles, matches, the sun, the sky, and many other things.
- In The People of Sparks, the Emberites don't know that the Earth is round, and don't know about things like animals, trees, trucks, rain, seasons, and gods.
- In fact, this is the whole point of The Giver. The title character's actual occupation is "Receiver of Memory," and his role in the community is to be the only one with access to Lost Common Knowledge (such as the existence of death, color, and animals), in case it comes in handy to the regime. For example, the Giver tells Jonas that he was consulted when the powers that be were considering increasing the number of children born, and he said no because he knew why they'd started controlling population numbers, to avoid starvation. Of course he and Jonas come to feel that everyone else is being deprived of the knowledge of good things and that they're unfairly burdened being the only ones to know about things like starvation.
- Taken to an extreme in Ayn Rand's novella Anthem: the protagonist rediscovers the very concept of individuality and the word "I".
- In the dystopian world of the Delirium Series, where love is considered a disease, the protagonist Lena does not know what poetry is. She considers it to be incredibly beautiful when she hears it, however.
- There is a early-2000s children's novel called Operation Timewarp in which five modern-day friends are selected for a La Résistance mission in early 22nd-century Britain. Once one of their close confidants finds out when they're actually from, he questions them about motor cars ("with pollution coming out of the back and people getting hit all over the place"), and "violent" sports (such as hockey and rugby).
- Civilization has (sort of) recovered by the time of The Wheel of Time series' protagonists, but periodically they come across ideas or information that strike them as absurd. One villain is trying to distract one of the heroes, for instance, and mentions off-handedly that in her time, people were able to travel to other worlds, "even worlds in the sky". Only the last bit fazes Nynaeve, who's pretty familiar with other dimensions — but worlds in the sky? That's just ridiculous. Bonus points to the Aes Sedai who have spent thousands of years trying to figure out how ancient people could fly. They're certain it must have been a magical effect, but no-one has ever managed it. It's on the verge of becoming discredited as a myth, possibly a mistranslated text. Perhaps the ancient Aes Sedai could fly that way, but only the readers (and a few Aiel, who don't become Aes Sedai) know they had airplanes.
- An Instinct for War features a future Forever War in which American soldiers sees a page from Clausewitz's On War and dismisses it as being enemy propaganda as they can't read it.
- The Dark Tower is set in a world that has "moved on", having suffered some never-specified apocalypse and slowly decaying. During a brief confrontation between Roland and the Man in Black early in the series (near the end of The Gunslinger), the Man in Black makes mention of the great deeds done by their progenitors; among other things, they cured cancer and walked on the Moon. Roland flatly refuses to believe the latter.
- In Sheri S. Tepper's The Visitor, the inhabitants of Bastion have been trying to rediscover the Lost Arts. However, they assume these were magic arts. Thus they explain science in magical terms. Actual science is known as Scienceism and considered heresy.
- In The Lost Fleet Captain Geary is periodically stunned when confronted with traditions, regulations, or beliefs that are not only contrary to his era but also just plain wrong. The biggest example, and what makes Geary so deadly, is the art of coordinating fleet-wide maneuvers at light-fractional velocities. Most officers learned this from more experienced officers before the war, but the massive casualties suffered by the fleet eventually wiped out anyone who knew.
- Eternity Road. According to legend the October Patrol used a submarine to salvage human literature and remove it to a place called Haven before society collapsed completely from The Plague, thus preserving human knowledge and culture for future generations. The plot involves an expedition to find this Haven, however their scholars scoff at the idea of a ship that can travel underwater, regarding it as a fanciful embellishment by later storytellers. Because of this they fail to realise that two of the doors at Haven form an airlock to a submarine docking chamber that has now flooded.
- An In-Universe case occurs in the original Dragonlance trilogy. On the edge of death, Raistlin frantically goes through the library of Astinus, trying to find a method in the books to save his life. He seems to have everything he needs except "The Key" which book after book mentions and the ancient mages apparently all knew, but none of the books actually explains what the Key is. When Raistlin realizes that none of the books will do so and the knowledge is apparently lost to time, he pretty much throws a fit.
- In The Stars My Destination, the protagonist initially can't get rid of the hideous tiger facial tattoo he got at the hands of a Cargo Cult because in the future tattooing became a lost art. He does manage to get it removed eventually but even that process is flawed since the tattoo reappears whenever he gets angry.
- By the Waters of Babylon: After a war had destroyed the US long ago, the future tribal humans no longer know much of how people lived then (they even think those once living were gods). John goes inside the former New York City, referring to "god roads" (probably paved ones) he traverses, and is unable to figure out how food they had is still in some cases edible through preservation, thinking they had magic containers. He eventually does realize they were not gods however, just humans, optimistic that through reading those books that are left behind eventually humanity can learn what they did and create it again.
- "The Quiz Broadcast" on That Mitchell and Webb Look is a post-apocalyptic game show that plays this for laughs. For example: "Pre-Event sources talk about "hope". What was hope?" Note that it is made clear that the game show takes place not that long after the Event. That's right: the Event was so horrible that people lost common knowledge of things they'd personally experienced for the bulk of their lives.
- In Star Trek: The Original Series, the episode "For The World Is Hollow And I Have Touched The Sky", the Fabrini built a generational starship into an asteroid to escape their dying homeworld. 10,000 years later, their descendants are completely oblivious to the fact they are living in space at all. Of course, it helps that the ones in charge make sure no-one finds out.
- Star Trek in general sometimes does this, with knowledge having been lost after the world war that devastated the earth and the cultural transformation brought on by replicators, the Federation, and alien influence. Unfortunately the writers don't always remember what things have been lost, so that the phrase "room service", which has been used several times in the shows without any hint that it's some archaic phrase, in one episode leaves the crew standing around in confusion until Data tentatively suggests that it's about cleaning the room.
- In the Doctor Who episode "The Space Pirates", Zoe, being from the futuristic twenty-first century, doesn't know what a candle is. (Though she'd had no trouble recognizing them in previous episodes).
- An episode of Blake's 7 had the protagonists arguing over whether all humans came from Earth originally. Given that the series used both Human Aliens and Future Imperfect tropes, either argument is plausible.
- In the The Twilight Zone (1959) episode "Elegy," a trio of astronauts lands on a planet that resembles Earth in the late 20th century, despite it being the year 2186 and them being in another solar system. Their landing site is a farm, complete with trees, a barn, hay, and a machine that Captain Webber and Kirby don't recognize. The third among them, Meyers, explains that it's called a tractor, a device that was in use on Earth before World War III.
- The Shannara Chronicles: Eretria scoffs at the idea Humans ever went to the moon, treating it as a myth.
- Utopia Falls:
- So much was apparently lost as a result of the war that in the future no one, even young black people, knows what hip-hop is at first.
- Apollo learns that the Indigenous drumming he sees has been passed down to him by his father, but never knew until then of his heritage. From seeing this he realizes he's Indigenous..
- In the RPG Paranoia, most inhabitants of Alpha Complex display this attitude about anything relating to the semi-mythical Outdoors.
- Warhammer 40,000 is all over this trope. Since nearly every world in the Imperium is insanely specialized, consisting of one climate, or both, what's commonplace in one world may be totally unheard-of in another. Someone who grew up in an Underhive, miles deep beneath the towers of a continent-spanning city, may be awestruck at the thought of a "sky"; meanwhile someone who grew up in a farm-dedicated agri-world would boggle at the thought of a Forge World that is almost wholly dedicated to manufacturing. This doesn't even get into the Mechanicus having such a strangle-hold on all things science after so much was lost over time, nobody has any idea HOW all of the technology works only THAT it works, and this includes many in the Mechancium itself!
- The Matoran on the island Mata Nui used to live in the underground island-city of Metru Nui, but they lost all their memories of their old lives when the Makuta trapped them and wiped their memories so he could control them when they awaken, but his takeover resulted in Metru Nui becoming uninhabitable. The Toa Metru foil Makuta's plan and evacuate the Matoran to Mata Nui, awaken them by giving up their Toa powers, and guide them in tribal living as weaker Turaga. To keep the Matoran from venturing into Makuta's traps by trying to return to Metru Nui, they keep all knowledge of their former home and the vast universe beyond it a secret, including their own past lives as Toa. Thus when another team of heroes, the Toa Mata arrive, they are hailed as the only Toa to exist. The running theme of the series becomes the Toa Mata and Matoran uncovering their secret history and the Turaga fessing up to their benevolent lies.
- The Toa Mata, lead by Tahu, are an elite team tasked with awakening Mata Nui should he be struck down. Unfortunately, the capsules they traveled in malfunctioned, leaving them in suspended animation for 1,000 years, and the details of their mission became jumbled nightmares. Upon waking up, they find they have a legendary reputation to live up to, legends about themselves that they've forgotten. They even have to re-learn some of their own powers via the Turaga's guidance. Near the end of the story, they come across a different group of Matoran in Karda Nui who know more about the Toa's lives and former heroics than they do. Tahu is the only one who, over time, manages to unlock his full memory.
- The Matoran of the sunken city of Mahri Nui used to live on the floating landmass Voya Nui, until their homes collapsed into the ocean and the water's mutagen altered their bodies and minds. This caused most of them to lose their memories, so they built a sanctum where Matoran can go and record whatever scraps of memories they have, to piece together their past history.
- In Time Traveller's segment of The Cave there's a (rather empty) museum with several examples, overlapping with Future Imperfect:
- Keys — nobody knows their purpose.
- Buckets — known only by description, probably used in a brutal children's game.
- President Franklin D. Bon Jovi, who was involved in World War I.
- A few examples of lost (or misinterpreted) knowledge crop up over the course of Civilization: Beyond Earth. In most cases it's less that the knowledge is plainly lost, as that very few people bother to learn it. The Purity affinity naturally has a building (the aptly-named Terra Vault) and wonder (Deep Memory) that are dedicated to the preservation and exhibition of what does remain in hopes of averting this trope.
Samatar Jama Barre: [on Deep Memory] You may say: "I remember," but we must also remember what our parents remember, and what our parents' parents remember as well.
- Dragon Age: Inquisition: Almost everything about ancient elven society has been forgotten by modern elves, including the fact that vallaslin, face tattoos worn by all Dalish, were once slave markings. This is also a huge motivation for Solas's plan to restore the pre-Veil world, since he feels modern elves are shadows of what they once were, and is dismayed that most modern Thedosians (of all races) don't know anything of what had been common knowledge in his day.
- Horizon Zero Dawn: Humanity is in a primitive tribal state without much knowledge of the old world, even though you have these obviously futuristic machine animals stomping around. Figuring exactly out what happened in the past is a major plot point in the game. There are many examples throughout the game, but the most obvious is probably the Collection Sidequest where you find old coffee mugs for a trader. He is convinced they were used in some sort of elaborate hygiene ritual (possibly involving shaving beards), and scoffs at the idea of such fine ceramic being used for anything as pedestrian as drinking.
- Triangle Strategy: In the present day, the only supply of salt anywhere in the continent of Norzelia is the Source, a body of saltwater in the desert. The game's inciting incident is the discovery of salt crystals underground in a mine. The fact that salt crystals exist at all is a closely guarded secret known only to the most important people in Hyzante, the kingdom where the Source is located, who has spent centuries espousing that the Source is the only place salt exists anywhere on the continent. The Roselle originally come from a far away land located next to something called an "ocean," a huge body of salt water, but this knowledge has been mostly lost to time.
- Star Wars: The Old Republic: An absolutely tragic case where a Republic PC searches through apocalyptic logs to learn what happened to a colony that disappeared shortly after The Empire bombed Taris into a toxic waste planet. After generations of devaluing education to survive endless famine, disease, failing crops, low fertility, still-births, rackghoul plagues, and more, the last leader of the last generation (only ten people) can only remember:
"Promised Land" Leader: They say there used to be a city here. I know what a city is — lots of houses... We've stopped giving birth. The old records say it was... [slowly] "tak-sick radie-ashtion."
- In The Last of Us, while some modern technology still exists, mostly in the quarantine zones, Ellie has a hard time believing that ice cream trucks used to be a thing. Similarly, the Left Behind DLC has Ellie and Riley find an old photobooth that asks if they'd like to upload their photos to Facebook: they spend some time mocking the idea of a "book of faces".
- Half-Life: Alyx establishes that younger generations born following the Combine takeover of Earth have a pretty poor understanding of what life was like before the Seven Hour War. Russell has to explain the concept of livestock to Alyx before he can even begin to explain what a club sandwich is, and later has to do something similar with basic business ideas like the stock market. Alyx also understands few to none of the pop culture references made by older characters; Larry bringing up Mozart gets nothing but a confused "Who?" from her.
- In Xenoblade Chronicles 3, after the protagonists free a given colony from the Forever War by breaking their Flame Clock, subsequent sidequests show many of their commanders refocusing their efforts on new endeavours to help their people survive, as they can no longer rely on supplies sent from their respective Castles. As their lives up to that point were solely dedicated to training for and carrying out warfare, this leaves very obvious gaps in their skillsets once they no longer need to fight. For instance, in the specific case of Colony 9, commander Zeon endures unrest in his community as he attempts to resolve their food shortages by effectively rediscovering agriculture, and experiences several issues he hadn't anticipated while trying to establish his crop fields, such as blights and droughts. Additionally, in Colony Mu, they gradually figure out to breed and ranch Armus and Arduns.note Both of these colonies learn about these concepts from old books, and have to get to grips with their practice through trial and error.
- In Shrapnel, thousands of years have passed since the end of the world, and only a few longer lived individuals like members of the Fenri know of the existence of the sun, as the weather is so consistently terrible that people either live in Endless Winter on the dark side of the planet, or a Deadly Dust Storm on the light side.
Potato: Where is light come from?Nutmeg: Waaay up there in the sky. There’s something called "the sun".Potato: Where?Nutmeg: You can’t really see it ‘cause of the dust, but it’s there. Some of the old songbirds told me about it.
- Stand Still, Stay Silent is set in a world where Iceland, Norway, Sweden, Denmark and Finland are the only surviving nations as far as the characters are concerned. At some point, one of the mages ends up in a spirit world location that is set up just as it was Just Before the End, and the location's resident naturally offers him a brown-colored beverage that has to be either tea or coffee. Upon drinking it, the mage calls it "soup" and says it tastes gross.
- Logan's Tale: At one point while exploring the wasteland, Logan comes across a children's playground, and has no idea what he's looking at.
- In Peace on Earth, two squirrel pups ask their grandfather what "men" are, since humans have driven themselves to extinction.
- Often played for laughs in Futurama:
- "The Series Has Landed": Although the moon has long since been colonized, the site of the Apollo 11 landing has been forgotten. Fry and Leela only find it by sheer luck. A plaque inside suggests that it was restored some time between now and the 31st Century, then abandoned. In a bit of Fridge Brilliance, when Fry and Leela find the original Apollo 11 site, they take refuge in the ascent stage of the Lunar Module. A plaque on one wall states "Lander returned to this site by the Historical Sticklers Society" (since in real life, only the upper body of the Lunar Module launched back into space, leaving the legs behind).
- "Mother's Day": After all machinery has rebelled, Fry has to reintroduce the concept of the wheel to the others. Of course, Fry being Fry, he gets the shape wrong.
- "A Big Piece of Garbage": To stop a giant ball of garbage from hitting Earth, they need to hit it with another ball of garbage. Unfortunately, since all matter is recycled, garbage doesn't exist as such, so it's up to Fry to teach the world how to make it.
- "A Clone of My Own" has Farnsworth demonstrating to Cubert his failed attempt at a universal translator, which only translates into "an incomprehensible dead language":
- The Simpsons, "Lemon of Troy". Bart has an Imagine Spot where people from the future bring him back to life using "technology" (namely, shooting a ray gun at a piece of graffiti he's made). He wows them by taking out a yo-yo.
Man 1: What's normal to him amazes us.
Man 2: He will be our new god.
- As many historians and archaeologists can confirm, this trope is very much Truth in Television. There's a lot we don't know about everyday life and technologies of the past, and much of what we do know comes from experimental archaeology, not primary sources. This is because, through much of history, people thought that only "important stuff" should be written down, like scholarly works, famous literature, religious texts, and government documents. No one would spend a scribe's value time writing down something everyone and their mother could recite off of memory. There was also an element of elitism in this: scholars, the upper crust of society, though themselves above studying and documenting the knowledge of the unwashed masses. It was only around the time of The Renaissance that we began to realize the error in this, which is why the first article in the first encyclopedia was about bread, of all things.
- The Renaissance started with the rediscovery of much of the ancient Greek and Roman lost knowledge.
- Possibly the quintessential example is Roman concrete, the use of which can be seen at any surviving Roman road or non-urban structure (dams, bridges, etc.). ~2000 years later and it survives with minimal wear, structures still serving their purpose, yet the complete, working recipe for it remained lost to the ages. In 2017, scientists at the University of Utah rediscovered the exact process; we had the recipe, but before this, when scientists tried to follow said recipe, the results were different than the Roman examples, because nobody in Rome bothered to write down that you need to use sea water.
- Up until the 18th century, occasionally unearthed stone tools were explained either as a natural phenomenon (pottery shards grow underground like potato or beet) or magic (flint tools are created by gnomes or by a lightning strike).
- Drilling holes in stone tools without metal, using just sand and sticks or bones was demonstrated only in the late 19th century.
- The purpose of bâtons de commandement ("rods of command") or pierced rods. Those are palaeolithic artefacts (12-23 thousand years old) made out of a 15-20 cm Y-shaped piece of a deer horn with 1 or more holes drilled. Originally though to be a purely ceremonial sign of an experienced warrior. Current hypotheses include: a tool to straighten arrows or spears (by passing them through the hole), a spear thrower (tested, proven to work), a symbol of fertility, a dress clasp, a calendar (most rods are decorated with patterns), a leatherworker tool... and a dildo.
- How moai were moved. A lot of theories were proposed, including Ancient Astronauts, but experiments have revealed that these huge statues can be "walked" using ropes and the same leverage techniques commonly used to move a heavy barrel. Rapa Nui oral history always said the statues walked but this was thought to be figurative until the technique was rediscovered.
- The suspension system of pre-Roman British chariots that allowed enough precision for archers to shoot from a moving chariot on a bad road. Researchers who studied the Wetwang chariot burial around 2010 concluded that it was a wooden frame hanging from wooden arches on rawhide strips.
- "Ganosis", a coating to protect polished marble from the elements. Well known in Ancient Greece, involved wax and heat, but only a few sculptors knew the exact process. Lost at the time of Roman conquest, rediscovered only in the 20th century.
- Another Ancient Greek formula allowed ivory to be turned into a soft and malleable wax-like substance and also reversed the transformation. Also rediscovered only in the 20th century.
- "Greek Fire" (actually used by the Byzantines, who were Roman, but ruled over Greece and spoke Greek) is a fairly well-known incendiary substance that was used as a weapon. Its properties were recorded by contemporary writers, but not its formula. A number of possibilities have been put forward, and weapons with a similar effect are easy to make today, but the actual composition has been lost to history.
- Damascus Steel was created in ancient times to make swords and was legendary for its strength, flexibility, and sharpness, not to mention its beauty. Modern metallurgy can produce slightly better steel but the original method is lost to the ages. Today's scientists have even discovered carbon nanotubes in these ancient blades.
- Similarly, few historians bothered to record mundane "obvious" information like what life was like on a farm in ancient China, or the workings of a marketplace in the Songhai Empire; leaving modern day historians of Rome to learn about the gossip, vulgar humor, "night life" and general street culture from studying things like ancient Bathroom Stall Graffiti, the Sigil Spam of ancient mercantile groups and manufacturers, and lewd carvings on the walls of ancient alleyways.
- A similar rarely discussed bit of information about ancient Egypt is that the workers carving the hieroglyphics were a bunch of Deadpan Servile Snarkers and aspiring political cartoonists. There are more than a few hilarious, crude, and hilariously crude Easter Eggs, Creator In Jokes and MSTs hidden in corners of many tombs and monuments. These often give insight into their actual opinions about those whose grand histories they were recording, and expand our understanding about how much is false and what can actually be considered somewhat reliable.
- The Old Testament makes mention that the Philistines had books detailing their religious beliefs and that their culture and history were well known at the time. However, no detailed account of the Philistines, either written by themselves or another country, has survived into the modern age, with the Bible being the main source of information we have about them.
- The Pythia, better known as the Oracle of Delphi, was the high priestess of the Temple of Apollo, the most authoritative oracle in Ancient Greece and unarguably the most powerful position a woman could achieve in the classical era. However, despite the Pythia being one of the most well documented religious institutions of the ancient world, most of the Oracle's actual duties and rituals, as well as how they were selected for the position, were never recorded since authors at the time felt little need to explain such universally known facts.
- It's well known that the Roman army was able to rotate their troops through the front line, but nobody is sure of the details of how it was done in the face of the enemy. The people who wrote histories either didn't know, or considered it as obvious and non-noteworthy as which end of the sword you hold in weapon drills.
- The idea that Roman emperors decided a gladiator's fate by pointing their thumb up to spare them and their thumb down to kill them is a modern convention created by classicist artists in the 19th century. Roman sources say that "a turned thumb" was used to communicate such thing, but don't say how the thumb was turned.
- Metal dodecahedrons are commonly found in Roman sites from the 2nd to 4th centuries, but their function is unknown and they aren't found in either contemporary paintings or written sources.
- Roland is the protagonist of the epic Chanson de Roland, written centuries after his death at the Battle of Roncesvaux, and the most famous of Charlemagne's knights as a result. The earliest source on the battle and only non-literary source on Roland, the early 9th century Vita Karoli Magni by Einhard, says that one "Hruodlandus" was killed - along with "Eggihard", "Anshelmus", and "many other knights" that Einhard didn't name because he considered them common knowledge.
- We have evidence that the story of the pied piper of Hamelin is Based on a True Story. We don't know how accurate it is, however, because the oldest surviving mention of it simply says that "it is 100 years since our children left". The author no doubt expected any reader to know exactly what had happened.
- The Spanish wrote that Mississippian culture archers in the southeastern U.S. could perforate two layers of chainmail and shoot down a galloping horse. Archaeologists have also found evidence that the Mississippians used to make wooden or leather armor in earlier periods but abandoned them by the late Middle Ages, which is consistent with an archery revolution making armor obsolete. Nevertheless, no Mississippian arrows or bows have survived, and the tree species in the area have woods apparently too soft to accomplish this. It's been suggested that they hardened the wood with fire and used "cable-backed" bows similar to Inuit bows, but this is just hypothetical.
- Throughout the Western world, salt and pepper shakers are standard dining-table equipment. However, older sets — from as recently as the 1850s — include a third shaker. Nobody alive knows what was put in it: sources from the period when the three-shaker set was common just say things like, "The usual condiments," or "salt and so forth." Bill Bryson's book At Home discusses the matter in some detail and lays out some of the theories as to what the mystery substance might have been.
- If it wasn't properly documented, something as mundane as the source inspiration or etymology of a city name can be this after several centuries/millennia. For example, there's a lot of proposed theories about how London or Edinburgh got their names.
- The Novgorod birch bark manuscripts. Not only do they reveal a lot about the daily life of the people back then (starting with showing a level of literacy any scientist suggesting would have been mocked for a century ago), but their very nature was a complete surprise. It was believed no letter could survive there, because while the waterlogged soil of Novgorod is perfect for preserving organic material, no ink will withstand centuries in water. However, once they dug up the first manuscripts, it turned out they were all carved on the birch bark with bone or metal stilos (which were a case of the trope themselves; for decades, archeologists mistook them for nails or awls) and are preserved perfectly. Only a bare handful out of the hundreds are written in ink, and reading these indeed proved hard even with the most modern methods.
- One device that defied explanation for a while turned out to be for sharpening thorns that were used as needles in early phonographs.