From the summit of the Great Orme, the landscape looks as peaceful as it is striking – all rolling green hills and farmland stretching out to the blue Irish Sea.

But the headland that rises over Llandudno, Wales has a secret, one that lay buried for thousands of years.

More than five miles (8km) of tunnels run beneath the hill's surface. Spreading across nine different levels and reaching 230 feet (70m) deep, some are so narrow that only children would be small enough to access them.

These are the tunnels of a copper mine: one that was first dug out some 3,800 years ago and that, within a couple of centuries, was the largest in Britain.

"In the 1980s and 1990s, you went from no Bronze Age mines being known in Britain at all, to – suddenly – a lot of small mines being discovered," says Alan Williams of the University of Liverpool's Department of Archaeology in the UK, whose PhD research focuses on the Great Orme. "And then the star in the crown was basically the Great Orme, which grew to be much bigger than all of the others and, in fact, is one of the biggest in all of Europe. It turned out to be the Stonehenge of copper mining."

You could say there was a golden age of the Bronze Age of the Great Orme

In the last year, Williams' analysis of Great Orme copper ore – which he presented at the 2015 Archaeometallurgy in Europe International Conference and which will be published in a forthcoming paper – has confirmed that the mine produced so much high-quality copper for making bronze, some of it wound up as far away as France, Holland and possibly Denmark.

In what he calls a "CSI-type exercise", he has found that Great Orme copper matches that in bronze artefacts found in north-west Europe. To show this, he examined two variables that "fingerprint" the copper: the ore's trace elements, and the ratio of different lead isotopes.

"You could say there was a golden age of the Bronze Age of the Great Orme, when it supplied most of the metal ore of Britain," says Williams. "Not only that, but we've found it on the Continent, in Brittany, in Holland, and possibly elsewhere."

Just as extraordinary, however, is that while the Great Orme was the biggest copper mine in prehistoric Britain, it was hardly the only one.

The first major copper mine in the British Isles emerged at Ross Island, in southwest Ireland, around 4,400 years ago. One of the reasons we know this is that artefacts like bone tools and charcoal have been found in many of Britain's prehistoric mines, allowing researchers to use radiocarbon dating to estimate their age.

Making bronze required serious sophistication

Copper, the first metal that humans discovered other than gold, was highly desirable. But while it was flashy, it was too soft to be ideal for making tools or weapons.

Better was mixing in a bit of hard, brittle tin with the copper. This formed the ideal mix: bronze. It first reached the British Isles from elsewhere, but by about 4,100 years ago Britain was producing its own.

Making bronze required serious sophistication: not only technologically, but in terms of organising groups of people, communicating and making plans, sometimes over long distances. This helps explain why researchers think of the Bronze Age, which in Britain is usually said to last from about 4,000 to 2,800 years ago, as a time of social complexity and connections across communities.

Take the manpower that had to be devoted just to obtain and smelt copper. Aside from mining, you needed people to cut and collect wood to make fires for smelting and others to bring stones from the beaches and turn them into tools – among other responsibilities required.

Meanwhile, the only place in Britain where tin is mined is Cornwall, a roughly 300-mile (480km) boat trip south from the Great Orme. And, as Great Orme manager Nick Jowett points out: "If you're going to go all the way to Cornwall, there's no point showing up at the beach with two tons of copper. There must have been meeting times. They must have communicated."

Ten Welsh copper mines as much as 4,100 years old have been found, says the Cambridge Archaeological Unit's Simon Timberlake. "Their working reflects a whole phase of looking for copper around the British Isles," he says.

However, he says, "a lot of these mines are very small, in fairness. They're nowhere near as big as the Great Orme."

By about 3,500 years ago, the last of these mines had petered out. Some may not have had rich deposits to begin with. Others were either exhausted or flooded when miners reached the water table.

Enter the Great Orme.

Like the other, smaller mines, the Great Orme got its start as a system of surface workings. Miners simply dug out the green and black veins of copper ore that they saw on the surface.

While there's a lot of copper in vast parts of the country, the Great Orme is special

But soon after, the miners decided to follow the veins of copper malachite both horizontally as well as down… and down, creating the winding, narrow tunnels that we see today.

The most intensive period of production was for two or three centuries around 3,500 years ago, although radiocarbon dating shows that the mine kept operating for another millennium.

"While there's a lot of copper in vast parts of the country, the Great Orme is special," says Williams. This was for several reasons.

Although copper can be smelted from a number of different types of ore, the Orme's mineral was a form of copper carbonate called "malachite" that was particularly easy to smelt.

It turned out to be the Stonehenge of copper mining

The veins were surrounded by dolomitic limestone. This is softer and therefore easier to excavate than other kinds of rock such as silica.

Since the Great Orme is situated in a headland, miners had a lot of rock to get through before they hit the water table. There were also natural cave passages that could be exploited.

But that did not mean mining was easy.

More than 2,500 stone hammers – the largest weighing 66lb (30kg) – and more than 35,000 bone tool artefacts have been found in the Orme. As a result, we know that miners usually hammered away at the rock with stone until they could scrape it away with bone.

However, more than 40 fragments of bronze have also been discovered in the mine – as have markings in the rock that could not have been made from stone or bone. Evidence of bronze picks has also been found in mines from the same period in Austria.

More than 2,500 stone hammers and 35,000 bone tool artefacts have been found in the Orme

This suggests the miners also used bronze mining tools. It is not surprising that only a few dozen have been found: a prehistoric miner might well toss away a bone tool underground, whereas a bronze one could be melted down for re-use, so would be worth hanging onto.

Given the back-breaking work, it is little wonder that the tunnels are so small. You would not want to dig more than you had to.

"Yes, they're really hard to crawl through, but then I think 'Oh, how about hitting the wall in front of you for eight hours at a time with a stone hammer' – that's hard, too," says Jowett with a laugh.

As for the tunnels that remain too small for even the most agile adults, these were probably excavated by children.

Family groups likely worked the mines, Jowett says. "Children were probably scraping out these veins while their parents were nearby. It was a different time. There's no school to go to. That's just the way."

As the miners dug deeper, they were in ever more risk of flooding. By the end, since the water levels changed seasonally, they probably would only have accessed the lowest levels in the summer. At 230 feet (70m) down, they hit the water table.

Children were probably scraping out these veins while their parents were nearby

Eventually the mine was abandoned completely. Within a few hundred years, Britain was once more getting nearly all of its bronze from the Continent, where miners were finding the veins even easier to exploit.

But in that few hundred years, the Orme had helped shape bronze networks across Europe.

No one can be sure exactly how much copper the mine produced. A number of factors make it nearly impossible to pinpoint, says Williams, including not knowing how much ore there was versus waste, and how much was lost during mining and smelting. As a result, estimates have ranged anywhere between 25 and 1,760 tonnes.

The more access you had to bronze, the more powerful you were

"Some of those higher figures have been criticised because it seems like far too much metal," Williams says.

It is hard to see what the small population of the British Isles could possibly do with so much bronze. As Timberlake points out, the combined metal contents of all the recorded Early Bronze Age axes from Ireland only amounts to 0.75 tonnes.

"But," Williams adds, "if you say that some of the metal was exchanged over long distances, that makes it more possible."

On mainland Europe as in Britain, bronze was used for everything from jewellery to spears. But among the most common finds are axe heads. Opposed to a fragile stone or a softer copper axe, a bronze axe would have made gruelling tasks like clearing fields, building houses or cutting firewood far easier and faster – making it the major multi-tasking technology of its day.

Because of its versatility and value, bronze helped shape society across Britain and mainland Europe. The more access you had to bronze, the more powerful you were, which archaeologists generally say helped underpin the era's hierarchical social systems.

Exchange of bronze also forged ties between communities, helping to spread ideas, new technologies and objects. And, ironically, that would come to include the spread of copper into Britain from mainland Europe – reversing the Great Orme's trend and ultimately meaning that Britain relied on trade with the Continent once more.

Not for another 1,500 years, with the arrival of the Romans, would metal mining become such a large industry in Britain again.

This story is a part of BBC Britain: a series focused on exploring this extraordinary island, one story at a time. Readers outside of the UK can see every BBC Britain story by heading to the Britain homepage; you also can see our latest stories by following us on Facebook and Twitter.

If you liked this story, sign up for the weekly bbc.com features newsletter, called “If You Only Read 6 Things This Week”. A handpicked selection of stories from BBC Future, Earth, Culture, Capital, Travel and Autos, delivered to your inbox every Friday.