Peter Ross: Can you dig it?

There's the castle, the Salisbury Crags, and Inchmickery island in the Firth of Forth. A buzzard soars through the azure sky, while rabbits jink through the grass.

The man I am with, however, does not look up. He keeps his eyes on the ground and his mind focused on the extraordinary sounds coming through his headphones. Alastair Hacket is a metal detectorist and his concentration - like the treasure he hopes to find - is deep.

Hacket is a slender man of 61 in a baseball cap and jeans. He lives in Edinburgh and is secretary of the Scottish Detector Club. There are around 200 serious detectorists in Scotland, and a few hundred more for whom it is a passing fancy. Anecdotal evidence suggests the economic crisis is causing a rise in public interest as people spend their redundancy money on detectors in the hope of finding treasure.

"You have to have a lot of patience," he says. "Keep going and going. Stick at it and eventually the good stuff will come up."

Just then, his detector beeps. It's long and black, shaped like a cross between a crutch and a giant's ski pole. It's worth around 600. You can spend three grand on a detector, but that's a bit mad. Hacket sweeps his machine back and forth across the grass, inducing further beeps, then presses a button marked "Pinpoint" which causes it to emit a continuous whine, increasing in pitch as the centre of the signal is located. Detectors work on the principle of electromagnetic induction. A signal transmitted into the ground induces an electric current in any metal object in the vicinity. The machine analyses the current to determine what sort of metal it might be. Experienced detectorists can interpret the sounds of the machine and judge whether it is worth getting their spade out. Right now, it's making a noise like an especially excitable bloodhound, which seems to mean there's potential.

"We'll dig this," says Hacket, "and see what it is." He rolls his eyes. "But it's bound to be junk."

The thing is, you never know. Metal-detecting, for years regarded as the niche interest of a few geeks with time - and mud - on their hands, has attained a higher profile and a degree of respectability over the past year, thanks to a series of astonishingly valuable finds.

In September 2009, David Booth, chief warden at Blair Drummond Safari Park, discovered four 2,000-year-old gold neckbands, known as torcs, on his first trip out with his metal detector. Earlier this month, he was back in the headlines again, this time for finding an 800-year-old silver seal with an inset Roman carving.

"Jammy bastard," jokes Hacket.

There's a fair bit of jamminess around at the moment. A huge collection of gold and silver Anglo-Saxon artefacts, unearthed in Staffordshire last year by an unemployed detectorist and valued at 3.825 million, was on Monday named Best Discovery at the British Archaeological Awards. On Thursday, a hoard of 52,000 Roman coins, discovered in a field in Somerset, was officially declared treasure, thus entitling the detectorist who found it to a reward equivalent to its market value, which could be around 250,000.

It would be wrong to say that metal-detecting is now cool. No hobby which has former Rolling Stone Bill Wyman as its poster boy could ever be called that. But detecting is certainly hot all of a sudden, and so I am keen to dig a little deeper and learn the truth about an activity which, till now, has largely been underground.

"Right, we've got something here," says Hacket. He has dug out a large divot from the field and is nosing around in the hole with a screwdriver-sized detector called a probe. "Oh, there it is. There's the culprit. We've got ... a watch. Not bad, eh?"

He turns it over and examines the back. "Well," he says, "it's not a Rolex."

In the world of metal-detecting, a watch, even a manky modern one without hands or a strap, is regarded as a relatively interesting find. The truth is that finding Iron Age jewellery or a fortune in Roman coins is very unusual indeed.

"You have to be realistic and just accept that it's not going to happen to you," says Hacket. "It's like the lottery. It always happens to other people. I do the lottery every week and I don't expect to hit the jackpot. If I get a small prize I'm happy, and that's a good analogy because I've found small prizes with the detector."

Among his most significant finds are two beautiful bronze Celtic brooches in the style known as dragonesque, which he sold to National Museums Scotland and swears did not pay for his kitchen extension. He also discovered, in East Lothian, four Bronze Age axe heads, which are thought to have been buried as a religious offering. That discovery was made six years ago and his excitement is still palpable.

"It's indescribable," he says, when asked how it felt. "A tremendous feeling. The first thing that occurred to me when I found the axes was I visualised a guy 3,000 years ago standing where I was standing. He knelt down, dug a hole, and put these things in, laying them at right angles to each other. It was mindblowing, spooky, to know that somebody actually stood on that ledge all that time ago."

It must be this sense of human connection that makes metal-detecting so satisfying. It's a coming together of past and present, a touching of hands and minds akin to Michelango's The Creation Of Adam. There's an emotional charge, too. Metal-detecting is based on loss. A misplaced ring which was a token of love. A dropped coin which might have bought a much-needed meal. There's a sadness in these small props from history which hint at a larger drama. You could argue, also, that metal-detecting contains within it comforting ideas of resurrection and salvation, the idea that nothing is gone or lost forever. If God has a hobby, it might be this.

Then again, it can be hell. Detectorists will go out in all weathers, often spending an entire day dodging cowpats while sweeping a field, dig something in the region of 500 holes, and come away with nothing but a few bent spoons.

Alastair Hacket is good enough to allow me a turn with his beloved machine. But all I find is tin foil. In fact, I am foiled at every turn. "This is how it goes. The farm worker takes the wrapper off his Kit-Kat and throws it away," Hacket says. "It's the bane of our life. The 20th century has a lot to answer for. So much junk. Irn-Bru cans are murder; you get a terrific signal off them."

In detectorist jargon, such disappointments are known as "heart-stoppers". You get a strong signal, dig down and see, through the mud, the glint of a gold disc. A sovereign, perhaps? But, no. It's the screw-top from a bottle of juice. Worth a few pence off a smoked sausage supper if you take it back to the chip shop, but nothing you could retire on.

The law in Scotland says that anyone who finds any historical object in the ground has no legal right to it. Neither does the owner of the land in which it is found. Finds should be reported to the Treasure Trove Unit (TTU) of the National Museums Scotland. The TTU assesses whether finds are worth claiming for the Crown; if so, a reward is usually offered and it is likely that this will be equivalent to whatever the item would fetch on the open market. Detectorist etiquette dictates that the money be split with the land-owner. You should seek permission from whoever owns the land before using the detector.

Although it is against the law to use a metal detector on a scheduled monument, for example an archaeological dig, it does go on, and some treasures have ended up on eBay. Known as "nighthawking", this practice has caused a great deal of bitterness between detectorists and professional archaeologists, leading to calls for metal-detecting to become licensed and regulated.

"Those people aren't metal-detectorists as far as I am concerned," says Hacket. "They are criminals."

He is concerned that nighthawking brings the hobby into such disrepute that the cultural benefits of metal-detecting are overlooked. "Some archaeologists feel that they are the only people qualified to pull stuff out of the ground, identify it and look after it," he says. "But a lot of the important stuff that's ended up in museums over the last 20 years or so has been directly as a result of metal-detecting and nothing to do with archaeologists. It wouldn't have been found otherwise."

Not that it's really about finding treasure and becoming wealthy. The point, I think, is to get out into the world, listen to the secrets of the earth, and enrich your present through a deeper appreciation of the past. "And anyway," says Hacket, "Ninety-nine per cent of what we find is crap."