Ten years ago, when metal detectorists were out near the village of Stixwould in Lincolnshire, UK, they turned up bronze fragments.

These turned out to be the remnants of not just one precious object, but many: swords, ferrules, and one spearhead after another. By the end, 129 bits of spearheads were found in the Tattershall hoard. All of the objects, researchers later determined, dated back to between 3,000 and 2,900 years ago.

Whenever anyone finds a group of prehistoric metal objects in Britain, they are legally obliged to report it to the Portable Antiquities Scheme. The officer assigned to the hoard, Adam Daubney, has handled a lot of discoveries. His Lincolnshire team alone has recorded 75,000 finds over the last 10 years. But this was different.

"The Tattershall hoard was a pretty special find," Daubney says. "When we see Bronze Age finds, they tend to be a fragment of an axe found in the landscape and nothing else. When you get a hoard, it flags up that something really special has happened in that part of the landscape."

The question, of course, was what. And it is a question that researchers across Britain and northern Europe are asking, not only of the Tattershall hoard, but of groups of metal objects that have been left in the ground across Britain. Many have lain undiscovered for 4,000 years.

After all, it seems odd that people would deliberately give up valuable items, especially those that have taken hours to craft.

While numerous hoards have been found from other periods of history – such as Roman coins and medieval jewellery – what is strange about the Bronze Age metal deposits is that they took off around 3,500 years ago, occurred for centuries and then, mostly, stopped.

When you get a hoard, it flags up that something really special has happened

From about 2,700 years ago to the arrival of the Romans around 43 AD – years we now think of as Britain's Iron Age – the trend for placing metal in the ground cooled off.

"We're dealing with about 30, 40 Bronze Age hoards a year. That adds up quite quickly," says Neil Wilkin, curator of the British Museum's British and European Bronze Age collections. "And that's just in England, not Wales, not Scotland. If you compare that to the Iron Age material, they probably have half a dozen [hoards] a year."

The majority of finds now come from metal detectorists, who – given the ubiquity of iron in today's Britain – often tune their gadgets to look for bronze, not iron. But even that cannot totally explain the difference, Wilkin points out, since people were still using bronze in the Iron Age.

"They just aren't depositing it a great deal," Wilkin says. "So it does seem like in the Bronze Age, compared to the eras that come after, they are depositing a lot of hoards."

But why?

For decades, archaeologists believed that Bronze Age people put metal objects in the ground with the intention of taking them back out later. After all, that is the idea behind the word "hoard": a group of objects set aside for a rainy day.

Why break your objects before storing them somewhere for safekeeping?

With some hoards, this may have been part of the motivation.

For example, the largest hoard of Bronze Age objects ever found in Britain, at Isleham in Cambridgeshire, included more than 6,500 objects from about 3,000 years ago. The objects included ingots and debris from metalworking, so researchers wondered if it was deposited by a smith who planned to retrieve and recycle the bronze later.

But other objects in the Isleham hoard complicate that idea: in particular, its tools and weapons, some of which were deliberately broken before being put inside the large ceramic pot.

Deliberately breaking hoarded objects was not unique to the Isleham hoard, either. Why break your objects before storing them somewhere for safekeeping?

Because of complications like this, researchers warn against a one-theory-fits-all method for Bronze Age depositions.

Further damage was carried out just before they were placed in the water

"It's tempting, and there are probably some overarching things we can pull out, but I'm quite sure that in different regions and occasions we had different meanings," Wilkin says.

For example, at one point it became de rigueur to put groups of ornaments in the ground. "There's a period in the middle Bronze Age when that's the thing to do, but mainly only in the south of England, and only for a couple of centuries. Then everything becomes about tools and weapons," he says.

Now that thousands of items have been found under Britain's Portable Antiquities Scheme, researchers have more data than ever before. This has led to an increasingly complex picture, but it has also showed some recurring themes.

One intriguing trend is that people often deliberately broke objects before they cast them off.

Take the Broadness hoard, made up of spearheads, a knife and a ferrule, which was found in the River Thames in Kent.

"Some of the notches and nicks on the edges of the weapons may be related to their use in battle," says Eleanor Ghey of the British Museum in London, who is researching British hoards as part of a joint project with the University of Leicester. "But we also know that further damage was carried out just before they were placed in the water, and this seems to be done with some deliberation."

We always get laughed at by the public for using the term 'ritual'

That is not the only premeditated aspect of Bronze Age depositions. People also tended to separate out the types of materials in the groups they buried. Rarely do you see gold and bronze deposited together.

These materials were also dumped in different places. Bronze hoards tend to be deposited in rivers or close to settlements, while gold tends to be away from settlements or field systems.

With all the deliberation involved, it is now thought that many of these Bronze Age deposits are much more than cast-off rubbish heaps for later recycling. Rather, the process of putting them in the ground was done in an organised, thought-through fashion.

In other words: a ritual.

"We always get laughed at by the public for using the term 'ritual'," Wilkin says. Without knowing what the ritual was for, the term can sound empty. "But from an anthropological point of view, ritual is a fundamental aspect of most communities, a constructed set of beliefs."

What is striking is how many different kinds of deposits were associated with water

In the same way you can see a process taking place in a graveyard and immediately recognise – from the step-by-step process, the symbols and the site – that you are watching a funeral, we do not need to know exactly why people threw precious items away to see that it had some kind of ritual purpose.

But just as a graveyard gives you important context about that funeral, so too does the landscape provide clues about the ritual's meaning.

In 2010, David Yates and Richard Bradley published a study of 30 finds of Bronze Age metalwork in East Anglia's Fenland area. They found a pattern.

Individual weapons were found intact in rivers, especially rapiers from the Middle Bronze Age and swords from the Late Bronze Age. Meanwhile, groups of bronze items, still mostly weapons, were discovered instead in pools or bogs.

We know there were rising water tables, increasing flooding, the backing up of river systems

"What is striking is how many different kinds of deposits were associated with water, and just how varied those findspots actually were," wrote Yates and Bradley. "Even a major find like the Isleham hoard was buried in a ditch terminal beside a former channel and a concentration of burnt flints."

There has long been a deeply-rooted relationship between water and the underworld: think of warriors being put into ships and the ships being burnt.

Maybe, Ghey suggests, weapons were broken and deposited after a warrior's death: for example, to allow the "killed" weapons to travel along with the spirits of the dead. "There are all sorts of theories," she says. "We'll never know the answer."

But the intensification of metal being put into watery places toward the end of the Bronze Age could also point to a different kind of ritual.

"There was a climactic deterioration," says Peter Chowne of the University of Greenwich in London, UK, an archaeologist specialising in prehistoric Lincolnshire. "We can see that with the dating of peat in Lincolnshire. We know there were rising water tables, increasing flooding, the backing up of river systems, all complicated in Lincolnshire by sea level change. That was definitely happening in the late Bronze Age, no doubt about it."

Perhaps the hoard's deliberately broken weapons mean it was a post-battle ritual

At the same time, many metal hoards in Lincolnshire appear specifically on the boundary spots between the increasingly flooded fens and higher ground.

The Tattershall hoard was found at one such place. Perhaps those who deposited it were trying to appease the spirit world, asking for a kinder climate.

Or, as one recent researcher of the hoard, Tobias Mörtz of the Free University of Berlin in Germany, has argued, perhaps the hoard's deliberately broken weapons mean it was a post-battle ritual.

The more data comes in, particularly now with the 20-year-old Portable Antiquities Scheme, the closer we should get to understanding why ancient Brits discarded precious objects. But we will never know for sure.

"It's like today: why do people go to church? There is a whole host of reasons. Some people have no faith whatsoever but they do so out of tradition. Others have an in-depth faith. Some go just for marriages, some go for deaths," Daubney says. "We're dealing with human beings and their mindsets, and that's never straightforward."

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