Last April, a man who hated history at school unearthed the largest coin hoard ever found in Britain. But why had it been buried in a field in Somerset?

Dave Crisp found treasure on a soggy ridge outside the Somerset town of Frome last April, and helped rewrite history. On a bitter winter afternoon, as he walks the frosty field again, he recalls one of the most heart-stoppingly exciting moments of his life. The 63-year-old ex-army man had discovered a scattering of Roman silver coins in the field. He came back a few days later with his detector, bought secondhand on eBay, to round up any remaining broken pieces. The signals were faint and confusing.

"I picked out a piece of Roman pottery, and when I turned it over there was a coin, a bronze radiate, stuck in it. When I turned over the next handful of clay, it was stuffed with coins — 20 at least. I just sat back on my heels and shouted: 'I've done it!'. I knew at once I'd found a Roman coin hoard in its undisturbed container — I knew the archaeologists would wet themselves."

He filled in the hole, chucking in an old horseshoe on the wild off chance that somebody else with a detector might happen on the site. Three days later, he returned with the professionals. His grandson came along for the fun of it, expecting to be clear by teatime: the two ended up sleeping in a tent to protect the deep pit, the find still only half exposed.

Crisp hated history at school, and left at 15 to join the services, where he became a cook. He now works as a hospital chef in Chippenham, and took up metal detecting as a hobby. The farm where he made his find, in a hamlet about a mile from Frome, is just an hour from his home in Devizes, and handy for a quick mooch about after an early shift. He talks easily of Roman emperors and Saxon kings, of gray ware pottery and silver siliquae coins, of the buckles and belt tags and strap ends which can light up this subject that now fascinates him.

What he found that afternoon is now stacked in a waist-high pile of shoe box-sized cardboard boxes, in a corner of an office in the coins department of the British Museum – a Diagon Alley place of mysteries, on two floors, protected by a three-inch-thick strongroom door.

The boxes hold the contents of a giant potbellied jar which lay in the clay of that sloping Somerset field for almost 2,000 years, filled to overflowing with the largest coin hoard ever found in a single container in Britain. "You can see what a job it's going to be to clean the horrors," Sam Moorhead, a Roman coins specialist, says fondly, running through his fingers a handful of disgusting bits of metal, green with corrosion, ragged with welded-on bits of other broken coins. Studying the 52,503 of them that are legible will occupy the experts for the rest of their careers.

Moorhead, and Roger Bland, another numismatist at the British Museum, scratch their heads over how to fund the work. Just washing and drying all the coins to prevent further corrosion, after they arrived in the museum, took two months. Their best guess is the full job will take three years and cost around £120,000 — but it could light up an obscure corner of Roman Britain.

The Somerset farm where Crisp made his find is clipped by a Roman road, but there is no record of a camp, villa, village, temple or cemetery anywhere in the area. But the Romans were clearly there.

When the amateur treasure hunter decided on that spring day to call in the professionals, he knew exactly what to do. Treasure finds must by law be reported — 806 in the just released figures for 2008. However, such discoveries are dwarfed by the torrent of objects, lower in value but priceless in history, reported in the 13 years of the voluntary Portable Antiquities Scheme, which has gradually built up a country-wide network of finds officers who record amateur discoveries. Both schemes are run from the British Museum, and headed by Roger Bland.

Before making his big find, Dave Crisp had reported scores of small finds from the same area of farmland. This time, the finds officer for Somerset quickly called in experts from the county heritage service; local archaeologist Alan Graham led the excavation joined by museum staff, Crips and his excited grandson, plus several members of the Sheppard family, the farm owners.

When Bland first got there and saw the deep pit, the broken empty pot and the mass of bagged coins, he admits his first reaction was, "Cripes, how are we going to deal with this?"

The 16 kilos of coins were moved to the British Museum for safekeeping and study, almost wrecking the suspension of Sam Moorhead's ageing VW. In July, as with all treasure finds, a coroner's inquest was held in Somerset, formally declaring the coins treasure. Then an expert committee met at the British Museum, and after hours of debate and three widely varying outside opinions on the value of the hoard, finally set a price of £320,250 on the coins, to be shared between Crisp and the landowners, Geoff and Anne Sheppard.

In the old days the British Museum would have wanted to keep the hoard. Instead, Bland and Moorhead are committed to finding the money in London for the conservation work and research, but are backing the Somerset museum in Taunton in its determination to acquire the hoard for its native county. The museum hopes to have done this by the time it reopens after a major redevelopment next summer; meanwhile, Crisp and the Sheppards have yet to receive a penny.

Britain for its size has more coin and other hoards than anywhere else in the Roman empire. The Staffordshire hoard, the largest hoard of Anglo-Saxon gold and silver metalwork yet found, made world headlines last year when it too was found by a metal detector in a nondescript field. The Winchester hoard – again found scattered across a field, 10 years ago – was several kilos of pure gold iron age jewellery. The Hoxne hoard, found in 1992 by a Suffolk farmer looking for a dropped hammer, held more than 400 pieces of gold and silver buried in the fourth century AD.

The conventional explanation is that these hoards are either underground piggy banks, or stashed in times of danger to be recovered when normal life resumes – or never, if the feared catastrophe overwhelms the owner. This fits some finds, such as the recent discovery in Somerset of English civil war-era silver, buried when the royalists were walled up in a nearby mansion with the parliamentarians on the march. But the Frome hoard doesn't match that picture at all.

When Moorhead and Bland sorted the coins they could identify, they turned out to have been minted for 25 different emperors, but from oldest to newest, they spanned just four decades. So these were not the accumulated savings of generations of local people.

The third century AD was not a time of trauma either. The Vikings were centuries away, the Irish behaving themselves, the Roman towns and cities growing a bit ragged at the edges, but the long rolling Somerset valleys were full of prosperous Roman villas and flourishing agricultural communities.

Nor will the piggy bank explanation work. The pot could never have been carried to or from the site full – the thin pottery would have collapsed under the weight. Within a few months, as damp and dirt seeped into the jar, if the people wanted their money back, they would have had to do just what the archaeologists did: smash the container.

The most recent coins in the hoard were minted for the near-forgotten Carausius, a bull-necked bruiser from Flanders, who was proclaimed by his soldiers emperor of Britain and Gaul when the Romans sacked him for looting the grain ships he was supposed to be protecting. Many of these coins were in superb condition, better – Moorhead says enviously – than those in the British Museum. They date the hoard to not earlier than AD293 when Carausius was murdered by his treasurer, but because they were in the middle layer, with older coins over them, they also suggest the pot was set into the ground and then filled in one load, not over a period of years.

And so, Moorhead is convinced, the only plausible explanation for how they ended up in the field is that the hoard was a ritual offering to the gods.

When civilians hear the word "ritual deposit" it may conjure romantic images of druids in procession, skin drums thumping and snake-shaped trumpets tootling. To many archaeologists, it suggests a despairing absence of other explanations. Yet Roman Britain abounded in gods. Every spring, rock, forest and valley, every season, every climate, was sacred.

Some of the names would have been familiar in Rome, but the Romans were also adept at incorporating much older beliefs: Sulis, the goddess of the hot springs at Bath, was a Celtic goddess who became Romanised as Sulis Minerva. When they were excavated, archaeologists found that the springs that fed the baths and the drains were full of coins, pieces of jewellery, even little prayers and curses inscribed on pieces of lead, addressed to the gods and thrown into the water.

"Nobody questions that before the Romans came, the people of Britain offered lavish gifts in metal to their gods," Moorhead says. "Why do we think that suddenly ended when the Romans came? If crops failed or dangers threatened, you made an offering for better times to come. If times were good – as they were in third-century Britain – you made an offering so that they would continue."

Moorhead is convinced the Frome hoard represents a stupendous offering of as much cash as the community could raise. The swords and bronze shields their ancestors threw into rivers and springs were gone, and coins were the easiest way of assembling a massive quantity of metal – and significantly, the Sheppards report that that ridge of their field is still so boggy in wet weather, it may well lie over an ancient spring. Future archaeology in the surrounding area may yet uncover more evidence of who lived there, and what they believed.

Dave Crisp is certain the ritual explanation is right. "There was something important to them about this place. Maybe there was an oak tree or a little sacred grove or a spring that's gone now. You can imagine a grandfather saying to the family 'that hill is special, that's where we always go'. Maybe times were bad, maybe times were good and they wanted to say thanks."

Since April he has been out with his detector on other farms, but found nothing except a few common coins. He looks forward to many more happy days after he retires next summer, with the new detector that has been his only extravagance since he learned the amount of the reward.

"Some archaeologists hate us," he says. "They'd really rather see this stuff left rotting in the soil. But it's our history waiting to be found and told, that's got to be right."