Drenched. Deflated. All words Gus Patterson and Derek McLennan might use to describe themselves at 3pm on December 19, 2013. The rain was lashing down, the field they were in was as muddy as a battlefield, and the 60mph winds were cutting through them. They had been there for five hours. In the diminishing light, hope had turned to hopelessness. It was time to call it a day. Then, all of a sudden, Patterson was dancing. Bending his knees and moving his arms as if doing a jig.

It is an unlikely image - that of the 49-year-old grooving to a non-existent soundtrack in an isolated field - until you understand why.

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It was a signal to McLennan, who was 40 feet away. But it was also an act of giddy, uncontrollable joy. McLennan would later describe Patterson as appearing to "bop away to some kind of techno-style Euro pop, with a little samba and line dancing thrown in." Patterson had found treasure. Honest-to-goodness, one-of-a-kind, buried treasure.

Patterson, nickname Ironman, and McLennan, aka Pokerman, are detectorists. The pair unearthed the largest medieval coin hoard found by detectorists in Scotland in Twynholm, near Kirkcudbright. Not since the time of Queen Victoria, they believe, has there been such a find; a collection of more than 300 hammered silver coins that include mints from the reign of England's Edward I, II and III and King Alexander III of Scotland, which dates to the early 1300s. Given that enthusiasts can hunt for 30 years without finding so much as a hammered coin (the most common form of early coin), to say their discovery is rare is an understatement. In the detecting world, theirs was not a once-in-a-lifetime find; it was a once-in- a-generation find. In today's money, it could be worth up to £40,000, although the reward will likely be valued at closer to £15,000.

Patterson, a social worker from Ayrshire, and McLennan, 46, a former vice-president of a Scandinavian energy firm, originally from Glasgow, are recalling their moment as we stand on a dirt road, surrounded by fields, minutes from the village of Hollybush near Ayr, close to where they live. They are dressed for a day of detecting but I have underestimated the elemental exposure. When Patterson lends me an oversized wind-breaker, I could not be more grateful.

Patterson recalls their find. "We had given up. So come three o'clock, I said, 'I'm ready for the road. I'm soaked to the skin.' I made my way towards the car. Then, 150 yards later, I got a signal, had a little dig. And up it popped - a silver hammered penny."

That is when he did his "hammy dance," as those in the hobby call it, a reference to hammered coins. McLennan rushed over and rubbed the mud from the coin, which turned out to be two stuck back to back. Both men went quiet. "This could be a hoard," McLennan whispered. Patterson's eyes widened. Within seconds, the pair picked up signal after signal. They were surrounded. They started scrambling in the mud, frantically digging. They were giddy with excitement. "Within an hour we had 40 coins," recalls Patterson. "If we find four between us in a year, we are happy."

When they went back to the steading in which they were parked, torchlight was upon them. "Who's there?" shouted the farmer's wife. "It's just us guys, detecting," they replied. "Ach, you'll no have found anything," she said. "Guys have done it before. Did you get anything?"

McLennan recalls his answer. "Well, in fact we did," he deadpans, the pair bursting into laughter in memory.

Calls had to be made. McLennan and Patterson, along with their four metal detecting friends, are called the Ayrshire Division, all of whom helped uncover the hoard in the following days. Together, they often attend "rallies", which can see up to 300 people detecting in one field, about 10% of which are usually women. In the days that followed, they dug from morning until night. Eventually word got out. Locals brought flasks of tea and soup and, before handing over the coins to the authorities, they displayed the hoard to a packed town hall. "It was unbelievable, a lifetime event," says Patterson.

Before we start detecting, Patterson and McLennan show me the other bits and bobs they have found. Under Scots law, everything found under the soil of historical interest is the ownership of the Crown. A declaration is filled out and finds are reported to the Treasure Trove Unit at the National Museum Of Scotland, where each find is assessed, often in a painstakingly-long process, to value its worth. If it is of value, a reward is recommended, usually at the going market rate.

McLennan has been detecting for two years; Patterson for half that time. Having met at a poker night a few years ago, the pair hit it off. Their time detecting - about twice a week, in all weathers - is as much about the banter as the possible bounty. If competitiveness exists, it's all in good humour. "It's kind of all-for-one and one-for-all," Patterson says. "For all that I found the hoard, both of us are on the declaration as joint finders."

Detecting has a reputation for attracting nerds - and is still far from being seen as a cool thing - but it seems to be on the increase. One theory is the recession - although it won't make you a quick buck. Detecting has its share of negative stereotypes; that those who participate are "land jumpers" - detecting on historical sites or on private land without permission - or "night hawkers" - whose illegal finds end up on the black market. "Unfortunately those people do exist," says Patterson. Like any hobby, there are unscrupulous people. But Patterson and McLennan are keen to dispel these negative images. They detect legally and responsibly. "We are up front with landowners. We tell them exactly why we are there."

McLennan produces a little box of finds that they have been allowed to keep. "You've got 1600-year-old Roman silver there. And this is an Elizabeth I penny," he says, producing a tiny coin, followed by an Elizabeth I shilling dating from 1560 and a Robert II groat. "That was found near here," says McLennan, before showing me a 22-carat-gold half sovereign of Queen Victoria.

I spy a gold band. Is that a wedding ring? "Yes, from 1824," he replies. "That has been a sore loss for someone because it has not been worn much. It was maybe lost picking potatoes."

The two know their stuff. McLennan has always had an interest in history and calls himself a "coin nerd". He methodically researches locations. Having found church tokens - formerly used by churches in exchange for communion - at nearby Balmaghie Church, when they handed them into the minister, they asked him if he had any other parishes. The man replied Twynholm. Because an agreement is in place with landowners to split rewards 50/50 - common practice in detecting - the church will also benefit from the hoard's eventual reward.

Patterson says he has also found musket balls, dating back to the 17th century. "Imagine that hitting you," he says.

McLennan produces a rare Chinese coin - likely dating to the 1800s when Scotland was trading with China - an antique brooch and an old tobacco jar lid. They are intriguing, these pieces, each with their own unique story, hidden underground. It is all part of the thrill of detection. One hole to the next, not knowing what they will find. Maybe something good, but more often not.

Before they know it, they have got another signal.

The machines - long, like ski poles, with a circular coil at the bottom - vary in price, up to a few thousand pounds. They can offer a reasonable idea of a find's size, shape and depth. But it need not be an expensive hobby. Machines can be bought second hand. As they sweep the ground, different tones indicate various types of metal, although they can be fooled by rubbish such as ring pulls. When the machines are on to something, the beeping, wailing response sounds like The Clangers. McLennan starts digging, but it is just an old belt buckle. "That goes in the scrap bag." He moves on.

Minutes later and his machine is going off again. He digs, reaches down and rubs the mud off his find. We lean in for a look. Silver glints. "It's a Queen Victoria threepence from 1893," he says, chuffed.

The shared fun of finding something - anything - is addictive. It is where the present meets the past. And the pleasing monotony of walking the fields, alone with your thoughts, is therapeutic. It's why Patterson likes it. "We go out together, but it's also a solo hobby. I can think things over and talk to myself and then we have a cup of tea and a blether."

"More silver!" shouts McLennan. It is a George VI shilling from 1942. "That's a happy day already," says Patterson. "We know guys that are manic about detecting. If they knew we had found that, they would be here like a light."

Patterson finds a "love token," a worn coin that men used to show their interest in a lady. They would bend them and give it to her and if she liked him, she would bend it too. "When you find one of those, some poor guy's been spurned."

Patterson says: "You never know what the next find is going to be." It turns out to be a fascinating Queen Victoria 60-year commemorative pewter medal.

McLennan tells me how Patterson once mistook a Viking cross for an old children's toy. It was 1000 years old, rare silver. The same thing happened with an antique baby brooch with gold inlay. "He thought it was plastic."

Given the success we have had this afternoon, the pair conclude the field was likely used in Victorian times for horse shows or fairs. On days when they find nothing, McLennan jokes that Patterson gets "metal-detecting depression".

"If it gets too cold and you are wet, then it can get miserable. But we always have a bit of banter so it does not bother us - although you see him dragging his spade," he says, laughing.

"That's why I follow you," Patterson replies. "I know the rubbish is cleared."

"I just wait until you find a silver coin, then I dig more of them," McLennan snaps back.

People hid their dosh as a primitive way of banking or for safekeeping if English soldiers were on their way. If they were off to fight, they would bury it in the hope of returning.

Every discovery - big or small - represents someone's loss; it's history unearthed. "For whatever reason - probably death - they never return. It would take a lot to stop anyone going back for their money."

What will happen to their hoard? "We would like it displayed in a museum, with our names against them. But that does not always happen."

They tell me the hours of research and money and time spent on finds are not always adequately acknowledged, even by Treasure Trove, although many finds end up in museums. "It is a hobby we take seriously," says Patterson. "But we feel our dedication is not given the recognition it deserves."

McLennan has written letters to Treasure Trove, not least because the two have yet to receive a form of receipt from them. Ultimately, the process is unclear, he asserts, adding: "The last words my dad spoke to me before his death were to wish me luck. He wanted the hoard to include the family name and be called the Patterson-McLennan hoard."

Unfortunately, from the letter exchanges, that is looking unlikely. "They (Treasure Trove) came back to me and said no. They said we will get a mention in their annual report."

That might be the only formal credit the pair receive. At the very least, reward aside, Patterson believes the hoard should acknowledge their names on it in a museum.

But at the end of the day, says McLennan, they are just happy finding anything.

"We cover every blade of grass," pronounces his partner, with a sweep of his arm. "We will return to these fields for the next year. If we don't hit it, it's too deep."

McLennan nods. "We are sure there is a hoard here. Whether it has already been found in Victorian times or whether it is still here to be found, you never know until you get out and walk the fields and try and find it. We have done it once." n