In an unheated school hall, on a damp winter’s evening, a group of men are strapping on riot gear, unsheathing blunt steel swords, taking out long-handled Dane axes, and going at each other hammer and tongs.

This is the London chapter of the Brotherhood of Jomsborg — “the world’s leading Viking organisation”, according to its slick website. To call it a re-enactment society would be to sell it short. To its most devoted members, the Brotherhood is nothing less than the modern reincarnation of the Jomsborg Vikings, a heavily romanced band of “bachelor bandits”, as one historian called them. They are said to have stalked the Baltic 1,000 years ago from their island base of Wolin in today’s Poland, though how much of their saga is history, and how much epic fiction, is uncertain.

Their latter-day namesakes are a mixed bunch. Simon, the new kid on the block, works for an online gambling company. Paul, a fair-haired, stick-thin Frenchman, is in travel insurance. João and his girlfriend Joana have come from Portugal in search of economic prospects. Tim and Joey are tradesmen, cocky and talkative: the likely lads. Alex is quieter, a rugby fan with relatives in the armed forces. Two more are policemen, one of them an alarmingly tall man who would look like a Viking heavy whatever he was wearing.

A few others turn up later. Long-haired Max runs an online store selling Viking merchandise; his business card reads “Live the Legend! Forge your Saga!” Norman, the head honcho, his knees too shot to fight these days, calls himself “the first black Viking”. A veteran of the re-enactment circuit, and one of the self-designated global Jomsborg elite, the Jomsvikings, he is now a gardener in Camden, helping to teach new guys his old tricks and reminiscing about past encounters. “Q”, only slightly less experienced and also a Jomsviking, is a British Muslim who fights not with one sword, but two, both curved in the eastern style, and eschews the need for a shield entirely. “Shields are for pussies,” he informs the company.

So much for anyone who thought the Vikings were blond, blue-eyed types. Welcome to the modern Viking world.

The Vikings of the past would have appreciated the diversity of their present-day acolytes. They were marauding raiders of Europe’s coastal towns and churches but also merchant-explorers of riverine Eurasia, canny traders with the Islamic Caliphate, hired swords in the service of the Byzantine emperor, stout farmers on treeless Iceland, beleaguered settlers on distant Newfoundland and founders of the first Russian proto-state, the Kievan Rus. Ever boastful in life, they would surely have loved their posthumous notoriety. About 1,200 years after they first sailed out of the northern mists and into our collective imagination, the Vikings are still there, refashioned for each new generation according to its tastes.

The historical Viking Age ended long ago, when European states hardened themselves against predatory attack. The Scandinavian raiding enterprises of the eighth century gave way to the staid Christian kingdoms by the 12th, and northern settlers in the British Isles and Europe were assimilated into, or overwhelmed, their host societies. Unruly pagan Vikings became good Christian burghers. An age of free-booting northern expansion was followed by one of royal consolidation. As Christendom’s attention turned east, towards the rising power of Islam, and its conquest of Byzantium and the Holy Land, the Vikings faded from the catalogue of European anxieties. For centuries they were dismissed as just another barbarian culture, surpassed by the march of progress and the rise of Christianity.

Yet today the Vikings are back, loud and proud. Their values, combining individualism with loyalty to the community, resonate with an age that has grown suspicious of state socialism and free-market capitalism alike. They loom large in historical documentaries (“the Hell’s Angels of the medieval age”, as one called them), in fictional television series, in films (“The 13th Warrior”, starring Antonio Banderas, is popular among aficionados), and in fantasy novels, notably the indestructible works of J.R.R. Tolkien. In Norway, there are plans to open a Disney-style Viking theme park in 2017.

They live on in national mythologies and stereotypes, in the coiling serpents of Viking tattoos and in supremacist ideologies that twist history and Viking spirituality to their own ends. (The Norwegian right-wing extremist Anders Behring Breivik declared himself proud of his “Viking heritage” before he went on his killing spree.) More happily, the Vikings appear in museum exhibitions from Kirkcudbright to Roskilde and in children’s books such as the one that first fired my imagination, “The Time Traveller Book of Viking Raiders” (c.1977). They reverberate through the echo-chamber of the internet. And they are here in the flesh, now, in a hall filled with the smell of men’s sweat and the folk memory of a violent past.

For two hours a kind of frenzied martial ballet takes place before my eyes as the Jomsborgs go through their moves, punctuated by the squeak of trainers on wood and the slap of swords on shields. Formations of fighters coalesce in battle, scatter and then dissolve as each combatant is forced to retire by a well-aimed blow or well-timed jab. Eventually the fighting comes down to two warriors, spinning around each other with their weapons at the ready. Fighting is “very cerebral, actually”, Simon says as he catches his breath between rounds. And there are rules: no hits to the head or the extremities. In Poland the battles are a little different, I am told, but then in Poland, one fighter notes, “it’s mad”. Here in London, health and safety come first. As Max helpfully informs me, “friends first, warriors second”.

In the pub afterwards, it’s isotonic drinks all round. Extraordinarily, several fighters are about to get in their cars, drive an hour to Slough and do it all again. For some the whole thing is lads’ stuff, with occasional paid work backing heavy-metal bands on stage. For others there is more to it. All the Jomsborgs have adopted Nordic names. Norman acts gnarly but is clearly proud of what he has achieved. “Q” has been doing Viking re-enactment since he was 17: “it’s my life”. Max and João both consider themselves Asatru or god-true, meaning that they adhere to a set of reconstructed pagan Germanic beliefs, a world of many gods governed by an ethic of courage, truth and honour.

We say goodbye on the pavement, shaking each other’s forearms, looking into each other’s eyes with expressions of eternal brotherhood and boundless loyalty. “Why don’t you come another Sunday?” one asks me. I thank them again, get on my bike and cycle off. Maybe.

In the beginning was the ship. Without these supreme symbols of technological proficiency and expansionist zeal — the Viking Age would not exist. To seek out the Vikings of the past, I have to cross the water myself.

On an autumn morning in Copenhagen, the courtyard of the National Museum of Denmark is crowded with children on school trips, the latest generation to be inducted into the Viking legend. The older ones loiter with attitudes of studied cool while they wait to go in; the younger ones scream their heads off. It is 20 years since the last big Viking exhibition in Denmark — and 30 since I was one of those screaming kids, at a similar exhibition at the British Museum in London, where this show will go after Copenhagen.

In Denmark, a new Viking exhibition is a national event. The foreword to the catalogue was written by Queen Margrethe herself, a trained archaeologist who also (under the pseudonym Ingahild Grathmer) illustrated the works of Tolkien. “Is there any period in our distant past”, she asks, “that fascinates us more than the Viking Age?”

I have come to see the centrepiece of the exhibition, a 100-foot longship known as Roskilde 6, the longest ever found. Just a quarter of its original wood remained when workers stumbled upon it in the 1990s at a museum building-site west of Copenhagen. Each piece had to be painstakingly rehabilitated, bathed in a solution of polyethylene glycol and then freeze-dried for months in a vacuum. “The wood remembers its old shape,” Kristiane Strætkvern, the Norwegian conservationist on the project, tells me. But to resurrect the vessel as a whole requires a modern steel skeleton on which to hang the surviving wood and give the ship its shape.

The final product is a terrifying, beautiful beast. A shallow draft allows it to sail swiftly and silently up river estuaries and disgorge its Viking crew straight onto dry land. Thin, narrowly overlapping planks of wood make it light and flexible, able to work with the waves rather than against them. A German animated film behind the ship shows thunder and lightning over a slowly undulating sea. “The video gives you seasickness just to look at it,” Strætkvern says. But I am already far away, trying to imagine what drove men to take to the storm-tossed seas for days at a time in the uncertain pursuit of glory, wealth or a new homeland. Roskilde 6 will never take to the seas in the same way again, but Strætkvern says she can make her boats sail in another way — in people’s minds.

It is not known for sure when or where this ship first pushed off from shore; the evidence suggests a date around 1025 and a southern Norwegian origin. Although it was patched up somewhere in the Baltic in 1039, we can only guess where else it sailed, who it was built for and why. Perhaps it was intended to weld together the realms of Knut, king of both England and Denmark — or perhaps it was commissioned by Norwegian warrior chiefs, itching to assert their independence.

We will never know the answers to these questions. But maybe they don’t matter. The mysteries of the Viking past are part of the secret of its appeal to posterity. They leave room for something more powerful than facts: imagination. The story of the Vikings can be endlessly retold, and no telling will ever be complete. The ship whispers to us still.

The Viking link across the North Sea is 12 centuries old. Perhaps older if you see Scandinavian expansionism as a mere extension of the marauding north German Angles, Saxons, Jutes and Frisians who invaded (or were invited into) post-imperial Britain a few centuries earlier.

But the use of the word “Viking” as a label to denote a unified Scandinavian culture, and describe a whole era of northern history, is more recent. To people of the early middle ages “Viking” was a job description: it meant sea-raider or, depending on your reading of the etymology, trader. As the British historian Eric Christiansen put it, calling the entire population of medieval Scandinavia “Vikings” is like calling all Americans cowboys. As late as the mid-19th century, Scandinavian scholars used the term sparingly and precisely. It was a Frenchman, Paul du Chaillu, who popularised the term “the Viking Age” by using it as a book title in 1889.

The V-word stuck, a shorthand too powerful to ignore, too commercially attractive. By the late 19th century the word had taken on a life of its own. Its overtones of ancient paganism — which made earlier generations of Scandinavians disinclined to apply it to their ancestors — became part of its appeal. The idea of hot-blooded pagan ancestors was attractive to a society emerging from an era of strait-laced Christianity: the more flamboyant, the better. Horned helmets emerge at this time — soon to become the bête noire of the purists, who like to point out that the Vikings didn’t wear them.

The European Viking boom was part of the same fashion for a half-imagined past that turned the relatively recent tartan kilt into the acme of ancient Scots’ menswear and made the myth-weaving Wagner Germany’s most celebrated composer. Across Europe, the pull of a dramatic, romantic history — deep in a country’s soil and blood — was irresistible at a time when ancient landscapes were being transformed by urbanisation and industrialisation.

For 19th-century Norwegians — long under the thumb of their fellow Scandinavians — Viking history provided a national story. When Norway became independent of Sweden in 1905, its new king took an Old Norse name: Haakon. Even today, Strætkvern notes, “the Viking Ship Museum in Oslo is laid out like a church.” For Danes, celebrating their Viking heritage became a way to embellish their freedom-loving history, expressing a sense of plucky resistance to their newly powerful Prussian neighbour. Later, in the 1930s and 1940s, the meaning of “Viking” culture took on a darker hue, subsumed by the Nazis into an emphasis on racial purity — and on the supposed common lineage of certain European groups.

Libertarians, proto-collectivists, anarchists, race-icons, risk-taking capitalists, saints and sinners: the Vikings have been pressed into service as all of these. Peter Pentz, curator at the National Museum of Denmark, sits in his book-lined office, surrounded by busts and portraits of his illustrious predecessors, and recalls his student days in the 1970s. The campus atmosphere of peace and love had seeped into the way people interpreted the Vikings, who were recast for the Age of Aquarius as sandal-wearing farmers and long-distance traders. The illustrators of my children’s book even gave them flared trousers.

Now, studies of the Vikings stress their interactions with the wider world, from Uzbekistan to America. Once nationalised, the Vikings are being globalised. And though there is still a popular association of Vikings with Nordic stereotypes, the academic focus is on ethnic diversity rather than genetic homogeneity. Someone even suggested that the Copenhagen exhibition should open with a blood test, under the banner “How Viking are you?”. The idea was swiftly dismissed.

But the emphasis on Viking warrior culture is back now too, a powerful draw for the young men who make up the bulk of the Vikings’ devotees. They feel the frisson of excitement and adventure that still clings to the Vikings, and gives an organisation such as the Brotherhood of Jomsborg its edge. “The most dangerous thing modern man does is get on his bicycle,” Pentz says, “but within us we have a dream of something more…dramatic.”

And perhaps we have become more comfortable with the idea that a society of the past may have been as contradictory as our own. The old dichotomy of us and them, civilised Christians and barbarian pagans, has broken down. Fighting is so foreign to our everyday experience that we can afford to fetishise it. “The Viking ship is a warship,” Pentz says sternly, “but it is beautiful; Viking poetry is about violence, but it is beautiful.”

Next morning, the smell in Peter Madsen’s office, above Copenhagen’s main shopping street, is a mix of freshly brewed coffee and antiseptic. Dressed all in black, set off with a grey scarf tied raffishly around his neck, Madsen is a thoroughly modern tattoo artist: intelligent, articulate and shaven-headed, with a fiery red beard.

He caught the Viking bug as a boy, getting into gaming first before graduating to building models for film merchandise. Coming back from Sweden a few years ago a friend suggested he try tattooing. “And here we are,” he says, now a successful businessman, with a job that is a cross between entrepreneur, micro-surgeon, artist and psychologist. “People open up when you’re drawing on them for two hours.” He once tattooed the same guy for 19 days. At least half his commissions are Viking, the continuation of an ancient tradition — the tenth-century Arab traveller Ahmad ibn Fadlan described meeting tattooed Vikings on the Volga river in Russia. On hearing that I am heading for the island of Bornholm, Madsen says: “there’s a tattoo needle in the museum there.”

People think of the Vikings as barbarians, he points out, “but their art work shows that that was not at all the case.” He scours images of Viking jewellery and runestones to emulate the masters. “People mix styles,” Madsen says amiably. “That annoys the fuck out of me.” White-supremacist appropriation of Viking symbols is just dumb nationalism, he says. “There are Muslim burials on Gotland [an important Viking trading emporium in Sweden]. That’s a hint.” The following week he is hosting a “Learn to Love the Swastika Week”. He senses my apprehension: “The Nazis took this symbol, dude.” Now it is time to take it back.

In the corridor Madsen shows me a map of the world with a pin marking the location of his customers. Most are in Europe, but some are as far afield as the American Midwest or East Asia. “There’s a subculture in Latin America where they sit around pouring mead down each other’s throats and shouting ‘Thor’.” So much for the tango.

Asked why Vikings captivate people all over the world, he has a simple answer: freedom. “All humans want experience, they want adventures, and Vikings do it in this awesome, powerful way.” To Madsen, Viking culture is about empowerment. “Don’t do anything just because.” Viking tattoos are about self-expression. Even Viking warrior culture is ultimately a way of finding oneself: “Combat and spirituality are very connected — adrenaline is a great kick to get your mind out there.” Viking war-fighting as the new yoga. Would the Brotherhood of Jomsborg see it that way, I wonder?

Madsen’s first customer of the day has been waiting a while. He goes out to greet her apologetically, taking both her hands in his. And then it strikes me. In another age he might have been a priest rather than a tattoo artist, attentive to his flock’s spiritual needs, reconnecting them through outward ministrations to their inner selves — and to their ancient past.

I bump down to earth on the island of Bornholm, half an hour’s flight from Copenhagen. Deep in the Baltic, 100 miles east of the rest of Denmark and due north of Poland, Bornholm was occupied by the Germans and then the Russians in the 1940s before becoming a cold-war listening station for the West. In the Viking Age it was a crossroads, a short journey from the Jomsborgs’ island lair at Wolin, at the nexus of trading routes stretching across northern Europe and into Russia — the very routes once taken by Roskilde 6. The soil of Bornholm is rich in Viking remains: more Arabic Viking-era coins have been found here than almost anywhere else. Something new is uncovered nearly every week.

The men — and it is mostly men — who make finds on Bornholm are not professional archaeologists or state employees but amateurs, armed with metal detectors and an intimate knowledge of the land underfoot. “Boys here get metal detectors for their confirmation,” says Finn Ole Nielsen, chief archaeologist of the Bornholm Museum and godfather to the island’s league of metal detectors. “We don’t want Bornholm to be a treasure island,” he adds. But metal detecting has yielded many finds. Local museums, to which all new discoveries must be reported, struggle to process them. So too the National Museum, responsible for distributing finders’ fees. The desk of René Laursen, a researcher at the Bornholm Museum, is covered in toothbrushes and callipers, clods of fresh dark earth spread out on pages of the local paper.

The museum’s display cabinets are full of wonders found nearby. Besides the tattoo needle Peter Madsen promised me, there is a stunning collection of over 2,000 delicate gullgubber — beaten pieces of fine gold imprinted with stylised animals and humans, tokens to give to the gods. And there are coins, lots of them. What about leaving things in the ground for future generations, I ask Frank Pelle, a retired IBM engineer? “You can’t empty the earth,” he assures me. But perhaps we should. Each year the ploughs cut deeper and chemicals leach further into the rich Danish farmland.

Other countries have been hit by the scourge of professional trophy-hunters donning infra-red goggles to dig illegally at the dead of night, selling their finds on the internet and destroying the integrity of the archaeological record. At one time, some feared such practices would come to Denmark: in the 1990s the cover of the archaeology magazine Skalk showed the Devil carrying a metal detector (banned in Sweden for years). But, besides the odd case of locals “forgetting” to register their finds, the Danish experience has been positive. “Perhaps it’s because we are a small country,” Nielsen says, “people feel that they work for the nation.” Anyhow, prestige ranks higher than money in the eyes of most metal hunters. They create virtual trophy rooms online. Amateurs can become more expert than university-trained professionals — a democracy of diggers.

“I do it whenever I can,” says Kim Lund Hansen, a young house painter. His breakthrough came in 2012 when, after scoping land with high concentrations of phosphates — a sign of previous habitation — he found a solid-silver 12th-century cross “as big as my hand”. Then, within a metre of it, he made another find, coins so fragile that he worried they would break. He phoned Nielsen, who was there in 20 minutes. “Now they call me the Knight Templar,” Lund Hansen says with a laugh. He bought his son a metal detector when he turned eight; the boy found a Thor’s hammer straight off.

The king of Bornholm’s metal detectors is Klaus Thorsen, dirt under his nails and passion in his eyes. His first find was an axe, staring at him from a lake when he was out fishing as a boy. Nielsen came round a week later to ask about his rumoured discovery. At first, he denied it: “I was afraid.” But eventually Thorsen relented — and he has not looked back. For 30 years he was a baker, waking early and using his free afternoons to fire up his metal detector. When he married, the National Museum made two copies of a cross he had found: “the only two in the whole world,” he says gleefully.

At heart, Thorsen’s story is a Viking one: of persistence, discovery and self-discovery. He still finds it strange when he stands up to speak and people listen to him. He has learned not from books, but from experience. “When you have found so much, you already have the data you need. It’s in here.” He taps his head. “We are not people who sit inside and read, we go outside.” Just like the Vikings.

The archaeologist, the hunter and I take a trip to the eastern side of Bornholm, to one of Thorsen’s favourite hunting grounds, within sight of his house. Nearby, a round, white, fortified church — a totem of Denmark’s post-Viking conversion to Christianity — stands like a lighthouse, visible for miles around, a reminder of the succession of spiritual meanings with which this landscape has been invested. “I’ve found so many things in this field,” Thorsen says, testing the solidity of the soil with his boot. He tells me about a time when he felt a tug on his shirt, like someone calling him to go out into this field. And how, when he got there, he found a gold figure under the only patch of ground not covered in snow. “I went up to it, and there it was,” he says, transported back to that moment of wonder.

The ancestors are still here. Walking by the shore as the winter sun begins to dip, Nielsen tells me how, on this very spot, the Vikings buried their dead, looking out to sea, defending their land even as they rested from their travels. And he tells me something I had, in my reverie, not fully realised. Every hidden hoard of coins found on Bornholm is a marker of a human tragedy: a family that didn’t live to use its buried loot, a warrior who died before he could get home, a trader robbed before spilling his secrets. This landscape that radiates peace and security was once scarred by war.

I stand a minute by the sea, half-expecting a Viking ship to appear around the headland, its ghostly crew prepared to disembark, arriving home after a long voyage. Waves roll in ceaselessly across the water, lulling me with their unbroken rhythm.

Back in London a few weeks later, I ring up Kristiane Strætkvern in Copenhagen to see how she — and Roskilde 6 — are getting on. The Danish exhibition has closed, but for Strætkvern and her colleagues a new phase of work has begun. They must oversee the painstaking work of disassembling the ship and preparing it for its next voyage: flat-packed into two lorries on the Esbjerg-Harwich ferry. I imagine the ancient Viking ship jammed between boxes of assemble-it-yourself pine furniture and crates of Scandinavian crime thrillers.

“A storm is coming in,” Strætkvern tells me, and it is threatening to disrupt the tight schedule for getting the ship ready to leave. But even Thor’s apparent displeasure cannot long delay Roskilde 6’s departure from Denmark and its journey to a new temporary home at the British Museum, where I first got the Viking bug 30 years ago.

I promise Strætkvern that we will look after the ship on its travels to London and Berlin. It is, after all, part of a common history, I say, a ship from a time when England and Denmark shared a king, ten centuries before, when the Vikings were known — and feared — all over Europe. “But no raiders this time.”

“Well,” Strætkvern replies, “let’s see.”

An image of Viking raiders flickers across my mind’s eye. My heart thumps. Nearly a thousand years after the end of the Viking Age, the Vikings, encoded in our cultural memory, can still produce a visceral shiver of fear. I think they would have approved.

Vikings: Life and Legend British Museum, London, Mar 6th to June 22nd; Martin-Gropius-Bau, Berlin, Sept 10th to Jan 4th