I januari 1997 besättningen på ett fiskefartyg i Östersjön hittade något ovanligt i sina nät:en fet gulbrun klump av lerliknande material. De drog ut den, placerade den på däck och återvände till bearbetning av sin fångst. Nästa dag, besättningen insjuknade i allvarliga brännskador på huden. Fyra fördes in på sjukhus. Den feta klumpen var ett ämne som kallas yperit, bättre känd som svavel senap eller senaps gas, stelnat av temperaturen på havsbotten.

I slutet av andra världskriget, USA, Brittiska, Franska och sovjetiska myndigheter stod inför ett stort problem - hur man kan bli av med cirka 300, 000 ton kemisk ammunition som återvunnits från det ockuperade Tyskland. Ofta, de valde det som verkade säkrast, billigaste och enklaste metoden:dumpa grejerna till havs.

Uppskattningar är att minst 40, 000 ton kemisk ammunition slängdes i Östersjön, inte allt i anvisade dumpningsområden. Vissa av dessa platser är markerade på sjökort men omfattande register över exakt vad som dumpades och var finns inte. Detta ökar sannolikheten för trålarbesättningar, och andra, kommer i kontakt med detta farliga avfall.

Problemet kommer inte att försvinna, särskilt med ökad användning av havsbotten för ekonomiska ändamål, inklusive rörledningar, havskablar och vindkraftsparker till havs.

Berättelsen om dessa olyckliga fiskare illustrerar två punkter. Först, det är svårt att förutsäga hur kommande generationer kommer att bete sig, vad de kommer att värdera och vart de kommer att vilja gå. Andra, skapande, att hålla och överföra register över var avfall dumpas kommer att vara avgörande för att hjälpa kommande generationer att skydda sig mot de beslut vi fattar idag. Beslut som inkluderar hur man gör sig av med en del av dagens farligaste material:högaktivt radioaktivt avfall från kärnkraftverk.

Den röda metallliften tar sju skakande minuter att resa nästan 500 meter ner. Ner, ner genom krämig kalksten för att nå ett 160 miljoner år gammalt lager av lera. Här, djupt under de sömniga fälten och tysta skogar längs gränsen till departementen Meuse och Haute-Marne i nordöstra Frankrike, franska National Radioactive Waste Management Agency (Andra) har byggt sitt underjordiska forskningslaboratorium.

Laboratoriets tunnlar är starkt upplysta men mestadels öde, luften är torr och dammig och fylld med brummandet av ett ventilationsaggregat. Blå och grå metalllådor innehåller en serie pågående experiment – ​​mätning, till exempel, korrosionshastigheten för stål, hållbarheten hos betong i kontakt med leran. Med hjälp av denna information, Andra vill bygga ett enormt nätverk av tunnlar här.

Det planerar att kalla denna plats Cigéo, och att fylla den med farligt radioaktivt avfall. Den är designad för att kunna rymma 80, 000 kubikmeter avfall.

Vi utsätts för strålning varje dag. Public Health England uppskattar att under ett typiskt år kan någon i Storbritannien få en genomsnittlig dos på 2,7 millisievert (mSv) från naturliga och artificiella strålningskällor. En transatlantisk flygning, till exempel, utsätter dig för 0,08 mSv; en tandröntgen till 0,005 mSv; 100 gram paranötter till 0,01 mSv.

Högaktivt radioaktivt avfall är annorlunda. Det är, först och främst, använt bränsle från kärnreaktorer eller rester som härrör från upparbetning av bränslet. Detta avfall är så kraftfullt att det måste isoleras från människor till dess nivåer av strålning, som minskar med tiden, inte längre är farliga. Tidsskalan Andra tittar på är upp till en miljon år. (För att sätta detta i något slags sammanhang, det är bara 4, För 500 år sedan byggdes Stonehenge. Runt 40, 000 år sedan, moderna människor anlände till norra Europa. För en miljon år sedan, kontinenten var mitt i en istid. Mammoter vandrade i det frusna landskapet.)

Vissa forskare kallar detta långlivade avfall "kärnkraftens akilleshäl, "och det är ett problem för oss alla – oavsett vår inställning till kärnkraft. Även om alla världens kärnkraftverk skulle sluta fungera i morgon, vi skulle fortfarande ha mer än 240, 000 ton farligt radioaktivt material att hantera.

För närvarande, kärnavfall lagras ovan jord eller nära ytan, men inom branschen anses detta inte vara en acceptabel långsiktig lösning. Denna typ av lagringsanläggning kräver aktiv övervakning. Förutom regelbunden renovering måste den skyddas från alla typer av faror, inklusive jordbävningar, bränder, översvämningar och avsiktliga attacker av terrorister eller fiendemakter.

Detta lägger inte bara en orättvis ekonomisk börda på våra ättlingar, som kanske inte ens längre använder kärnkraft, men utgår också från att det i framtiden alltid kommer att finnas personer med kunskap och vilja att övervaka avfallet. På en miljonårig tidsskala kan detta inte garanteras.

Så, efter att ha övervägt en rad alternativ, regeringar och kärnkraftsindustrin har kommit till uppfattningen att djupt, geologiska förvar är det bästa långsiktiga tillvägagångssättet. Att bygga en av dessa är en enorm uppgift som kommer med många komplexa säkerhetsproblem.

Finland har redan börjat bygga ett geologiskt förvar (kallat Onkalo), och Sverige har påbörjat licensprocessen för sin webbplats. Andra räknar med att ansöka om sitt bygglov inom de närmaste två åren.

Om Cigéo tas i drift kommer den att inrymma både högaktivt avfall och det som kallas medelaktivt långlivat avfall – som reaktorkomponenter. När förvaret har nått kapacitet, om kanske 150 år, åtkomsttunnlarna fylls på igen och förseglas. Om allt går enligt planerna, ingen kommer någonsin att komma in i förvaret igen.

Stå framför en oskärmad strålningskälla och du kommer inte att se eller känna någonting. Dock, en del av den strålningen kommer att passera in i din kropp. Kärnavfall är farligt eftersom det avger joniserande strålning i form av alfa- och beta-partiklar och gammastrålar. Medan alfapartiklar är för svaga för att tränga in i huden, betapartiklar kan orsaka brännskador. Vid förtäring, båda kan skada inre vävnader och organ.

Det är gammastrålar, dock, som har det största penetrerande intervallet, och därför potentialen att orsaka den mest utbredda skadan på dina cellers DNA. Denna skada kan leda till en ökad risk för cancer senare i livet, och det är till stor del ansvarig för den uppsättning symtom som kallas strålningssjuka.

Vissa experter uppskattar att en dos på över 1 sievert räcker för att orsaka strålningssjukdom. Symtomen inkluderar illamående, kräkningar, blåsor och sår; dessa kan börja inom några minuter efter exponeringen eller fördröjas i dagar. Återhämtning är möjlig, men ju högre stråldos, desto mindre troligt är det. Vanligtvis, döden kommer från infektioner och inre blödningar orsakade av förstörelsen av benmärgen.

För avfall begravt djupt under jorden, det största hotet mot folkhälsan kommer från vattenförorening. Om radioaktivt material från avfallet skulle blandas med strömmande vatten, det skulle kunna röra sig relativt snabbt genom berggrunden och in i marken och stora vattendrag som sjöar och floder, äntligen in i näringskedjan via växter, fiskar och andra djur.

För att förhindra detta, ett underjordiskt förvar som Cigéo kommer att vara mycket noga med att skydda det avfall det lagrar. Within its walls there will be metal or concrete containers to block the radiation, and liquid waste can be mixed into a molten glass paste that will harden around it to stop leakage.

Beyond those barriers, the planners choose their sites carefully, so they can exploit the properties of the surrounding rock. At Cigéo, press officer Mathieu Saint-Louis tells me, the clay is stable and has very low permeability, making it hard for any radioactive material reach the surface. After around 100, 000 years a few very mobile substances with a long half-life, such as iodine-129, might manage to migrate upwards in extremely small quantities, but at that point, Saint-Louis says, the "potential impact on humans and the environment is much lower than that of radioactivity that is naturally present in the environment."

Deep geological repositories are designed as passive systems, meaning that once Cigéo is closed, no further maintenance or monitoring is required. Much more difficult to plan for is the risk of human intrusion, whether inadvertent or deliberate.

In 1980, the US Department of Energy created the Human Interference Task Force to investigate the problem of human intrusion into waste repositories. What was the best way to prevent people many thousands of years in the future from entering a repository and either coming into direct contact with the waste or damaging the repository, leading to environmental contamination?

Over the next 15 years a wide variety of experts were involved in this and subsequent projects, including materials scientists, anthropologists, architects, arkeologer, philosophers and semioticians—social scientists who study signs, symbols and their use or interpretation.

Science fiction author Stanislaw Lem suggested growing plants with warning messages about the repository encoded in their DNA. Biologist Françoise Bastide and semiotician Paolo Fabbri developed what they called the "ray cat solution"—cats genetically altered to glow when in the presence of radiation.

Quite apart from the technological challenges and ethical issues these solutions present, both have one major drawback:to be successful they rely on external, uncontrollable factors. How could the knowledge required to interpret these things be guaranteed to last?

Semiotician Thomas Sebeok recommended the creation of a so-called Atomic Priesthood. Members of the priesthood would preserve information about the waste repositories and hand it on to newly initiated members, ensuring a transfer of knowledge through the generations.

Considered one way, this is not too different from our current system of atomic science, where a senior scientist passes on their knowledge to a Ph.D. candidate. But still, putting such knowledge, and therefore power, into the hands of one small, elite group of people is a high-risk strategy easily open to abuse.

Perhaps a better way to warn our descendants about the waste is to talk to them directly, in the form of a message.

At Andra's headquarters outside of Paris, Jean-Noël Dumont, head of Andra's memory program, shows me a box. Inside, fixed in plastic cases, are two transparent discs, each around 20 centimeters in diameter. "These are the sapphire discs, " he says. The brainchild of Dumont's predecessor, Patrick Charton, each disc is made of transparent industrial sapphire, inside which information is engraved using platinum.

Costing around 25, 000 euros per disc, the sapphire (chosen for its durability and resistance to weathering and scratching) could last for nearly 2 million years—though one disc already has a crack in it, the result of a clumsy visitor on one of Andra's open days.

In the very long term, fastän, these plans also have a major drawback:how can we know that anyone living one million years in the future will understand any of the languages spoken today?

Think of the differences between modern and Old English. Who of us can understand "Ðunor cymð of hætan &of wætan"? That—meaning "Thunder comes from heat and from moisture"—is a mere thousand years old.

Languages also have a habit of disappearing. Around 4, 000 years ago in the Indus Valley in what is now Pakistan and north-west India, till exempel, people were writing in a script that remains completely indecipherable to modern researchers. In one million years it is unlikely that any language spoken today will still exist.

In the early 1990s, architectural theorist Michael Brill sought a way to side-step the issue of language. He imagined deterrent landscapes, "non-natural, ominous, and repulsive, " constructed of giant, menacing earthworks in the shape of jagged lightning bolts or other shapes that "suggest danger to the body... wounding forms, like thorns and spikes."

Anyone venturing further into the complex would then discover a series of standing stones with warning information about the radioactive waste written in seven different languages—but even if these proved unreadable, the landscape itself should act as a warning. To help convey a sense of danger there would be carvings of human faces expressing horror and terror. One idea was to base them on Edvard Munch's The Scream.

The drawback is that such a landscape—a strange, disturbing wonder—would probably attract rather than repel visitors. "We are adventurers. We are drawn to conquer forbidding environments, " says Florian Blanquer, a semiotician hired by Andra. "Think about Antarctica, Mount Everest."

Or think about the 20th-century European archaeologists, people not noticeably hesitant when it came to opening up the tombs of Egyptian kings, despite the warnings and curses inscribed on their walls.

As Dumont sees it, a memory program is necessary for three main reasons. Först, to avoid the risk of human intrusion by informing future generations about the existence and contents of Cigéo.

Andra, to give future generations as much information as possible to allow them to make their own decisions about the waste. They might, till exempel, want to retrieve the waste because new uses or solutions have arisen. Gerry Thomas, chair in molecular pathology at Imperial College London, believes that much of the waste destined for repositories may one day provide an important new non-carbon fuel source.

Tredje, cultural heritage:a properly documented geological repository would provide a wealth of information for a future archaeologist. "I have no knowledge of other places or systems where you have at the same time objects from the past and very large, concrete descriptions of how these products were manufactured, where they come from, how we considered them and so on, " says Dumont.

One way that memory is transmitted is orally, from generation to generation. To study this, Dumont asked researchers to consider historical examples of oral transmission, using as a case study the 17th-century Canal du Midi between the Mediterranean and Atlantic Ocean. Här, for 300 years, the same families have worked on maintaining the canal, passing down know-how from father to son.

Dumont also talks about the need to ensure that as many people as possible hear about Cigéo. As part of this strategy, Andra has held a series of annual competitions asking artists to suggest ways to mark the site. Till exempel, Les Nouveaux Voisins, winners of the 2016 prize, imagined constructing 80 concrete pillars, 30 metres high, each with an oak tree planted at the top. As the years passed, the pillars would slowly sink and the oak trees replace them, leaving tangible traces both above and below the repository.

Leaving Andra's visitors' center, I drive through a landscape patchworked with colors, from the russet of the woods to the bright limey green of a wheat field, towards Bure, a tiny village of around 90 inhabitants. The population is aging.

"Young people can't stay here if they want to study and find jobs, " Benoît Jaquet tells me. A village that once supported around ten farmers is now home to only two or three. Although not a resident of Bure, Jaquet is the general secretary of CLIS, an organization of local elected officials, representatives from trade unions and professional bodies, and environmental associations. Its purpose is to provide the local community with information about Cigéo, host public meetings, and monitor the work of Andra by, till exempel, commissioning independent experts to review the agency's work.

If the repository is built, Jaquet says, French law requires that CLIS be transformed into a local commission that will last as long as the repository. "So it's also a way to pass the baton, " he says. "If there is a local commission there is a memory—not Andra's memory but an external memory."

På samma gång, Andra has set up three regional memory groups, each composed of around 20 interested locals. They meet every six months and make their own suggestions for passing on the memory of the repository. Ideas so far include collecting and preserving oral witness accounts and developing an annual remembrance ceremony to take place on the site, organized by and for the local people. A nuclear beating the bounds, a radioactive summer solstice, an atomic maypole.

This last idea resonates with the work of Claudio Pescatore and Claire Mays, former employees of the Nuclear Energy Agency, a Paris-based body that supports intergovernmental cooperation on nuclear issues. They wrote in a research paper:"Do not hide these facilities; do not keep them apart, but make them A PART of the community… something that belongs to the local, social fabric." They went on to suggest that a monument celebrating the repository could be created, and argued that if it had "a distinctiveness and aesthetic quality, would this not be one reason for communities to proudly own the site and maintain it?"

Could the repository, I ask Jaquet, one day become a tourist destination? On the contrary, han säger, some members of the CLIS say that "every person living here will quit the district because of the risk, because of the image of the repository as a rubbish bin. Of course some also think the repository will create employment and that this will become a new Silicon Valley. Maybe the reality will be somewhere between the two—but a tourist attraction? I'm not sure about that."

Across the road from CLIS and the town hall is a large, ramshackle stone house decorated with a banner. It translates:"Free zone of Bure:house of resistance against nuclear waste." Since 2004, this has been home to a rotating group of international anti-nuclear, anti-repository protesters. By continually campaigning against Cigéo—and, presumably, by passing their beliefs on to future generations—the protesters would necessarily keep the memory of the repository alive and in the public eye, the ramshackle stone house becoming its own sort of monument for Cigéo.

"So in fact the pro-repository groups need the anti-repository groups to stay alive in order to provide a good memory, " says Florian Blanquer. "Fortunately, we are in France—in France there are always opponents to something!"

Rely only on the transmission of knowledge between generations and you can never guarantee an unbroken line of succession. Rely only on direct communication and you risk leaving behind a message that, even if it survives physically, eventually no one will be able to understand. So Andra asked Blanquer to research how to convey a message without written language.

Many visual signs are, like languages, culturally specific. Vidare, we know that the meanings of signs are not always stable over time.

Fortfarande, Blanquer thought that there was one universal sign:an image of a human figure. "And every human being… apprehends its body through space the same way as well. There is an up and down, a left and right, a front and back, " he wrote in a conference paper. Pictographs (pictorial symbols for a word or phrase) based on an anthropomorphic figure in movement are likely to be recognized universally, he decided.

Now he had the beginnings of an idea, but it wasn't enough. You might draw a cartoon strip showing a person approaching a piece of radioactive waste, touching it and falling down. But how can you guarantee that the panels will be read in the correct order? Or that touching the waste will be interpreted as a negative action? And how can a pictograph relying on the visual representation of tangible objects convey a message about radioactivity—something that can be neither seen nor touched?

In response to these problems, Blanquer has designed what he calls a "praxeological device." Independent of any verbal language, it works by teaching the person encountering it a brand-new communication system created specially for this purpose.

Blanquer envisages a series of passages built underground, perhaps in the access tunnels of the repository. On the wall of the first passage is a rectangular pictograph showing a person walking along the passage and a line of footprints indicating the direction of movement.

At the end of the corridor is a hole and a ladder and three more pictographs. A circular pictograph shows a person holding on to the ladder; a triangular pictograph shows a person not holding on and consequently falling off. And so it continues.

In this way you begin to establish patterns:you learn first that the figure drawn on the walls relates to a person's actions here, and second that you should copy the actions in the circles and avoid the actions in the triangles. "What is really interesting is the idea of people learning by themselves, " Dumont says. "Learning is important in the long term when you cannot just rely on transmission from generation to generation."

There has been one more radical proposal about how to deal with the threat of human intrusion—hide the repository completely from future generations.

Some argue that because the repositories are passive systems, most likely buried far underground in areas with no deep natural resources, the question of memory preservation is moot.

För närvarande, no one can conceive of a reason why anyone in the future might want to dig down 490 meters to reach the clay formation that Cigéo is planned for. This reduces the chances of inadvertent intrusion. And after around, say, 100, 000 år, almost all surface traces and any complex above-ground markers will have vanished. The only things left behind will be some slight indentations, perhaps a gentle protuberance or two. Things that to the untrained eye may appear to be only the natural shape of the land. Eventually it will be as though no one was ever there, as though there is nothing for anyone to remember.

But Blanquer warns that forgetting is not so easy:"You cannot say to yourself, "I will forget about that." It's like trying not to think about pink elephants. If you want to forget about it then first you have to get rid of any information about it. That would mean shutting down the web and destroying a lot of computers, a lot of newspapers, a lot of books."

In his opinion it is no longer possible that Cigéo could become, as Danish film maker Michael Madsen has said about the Finnish repository, "the place you must always remember to forget."

Last summer I set out with some friends to walk part of the Ridgeway, an ancient long-distance route through the Chiltern Hills and North Wessex Downs in the south of England. On Whiteleaf Hill, the chalky white path passes near the remains of a Neolithic barrow, around 5, 000 år gammal. You can tell immediately that it's not natural, the way the earth has been lumped up on the hillside, but today there is little to see except a low grassy mound with a view over the fields and woods of Buckinghamshire and the small town of Princes Risborough.

We don't know who built the burial chamber or the name of the person interred there, what language they spoke and what they believed the world would be like in 5, 000 år. Staring at the barrow, it was not continuity with the past I felt, but distance.

In the 1930s an archaeologist called Lindsay Scott broke open the Whiteleaf Hill barrow and discovered the remains of a human skeleton, around 60 pieces of pottery, flint shards and animal bones. And just as we enter burial chambers in search of answers, so archaeologists of the future may one day find themselves penetrating the concrete passageways and tunnels of the place we call Cigéo.

Peering into the darkness they will ask themselves, who built this place and why? Why did they come here, digging down so far below the surface of the land? What were they running from, or trying to hide?

In the light they carry, the archaeologists will see markings on the passage walls. Moving closer, they make out a series of footprints stretching away in front of them, down the passageway. In the looming darkness, it becomes clear—someone has left them a message.

This article first appeared on Mosaic and is republished here under a Creative Commons licence.