It's an old story, and a very old quest. More than 2,000 books have been written about the lost island civilisation described by Plato. Scientists and cranks, mystics and opportunists and many others in between have been seeking and speculating about Atlantis. The island has been located in places as far apart and improbable as the Antarctic and the South China Sea, Scandinavia and the Azores. The floor of the ocean in the rough area where Plato apparently located it, in the Atlantic west of the Straits of Gibraltar, known to the ancient world as the Pillars of Hercules, has been well scoured, without success. What's different about the latest attempt is that the man behind it, a 39-year-old Iranian-American called Robert Sarmast, displays no doubts: he is absolutely convinced that he has discovered Atlantis. This month on his website, discoveryofatlantis.com, he published three dimensional images which he claims prove the existence of the acropolis of Atlantis, seven kilometres off the Cyprus coast and 1,000 metres below the surface.

He has persuaded a mainstream American documentary maker, TMC Entertainment, based in Los Angeles, to climb on board, and sink tens of thousands of dollars into making a two-hour live special on Mr Sarmast's final expedition and the "filming of the structures" next year. "This TV special, says the company, "will enable viewers worldwide to participate in the thrill of discovery as they watch, live on their own TV screens, as manned submarines film underwater ROV submersibles blasting sediment off the buried structures - revealing the full detail of what has lain hidden for probably more than 12,000 years." TMC producer Drew S. Levin said: "We are absolutely thrilled to be associated with what may in fact be the greatest archaeological discovery of modern times. All the indications are that Robert and his team of highly-credentialed researchers have indeed found the acropolis of the lost Atlantis ..."

Mr Sarmast has described what he is going to reveal to the world so vividly it is as if we were already standing, blinking, before it. He believes that Cyprus was merely the highest mountain range at the north-western tip of Atlantis. The ancient land itself spread eastwards towards what is now Syria. He says: "Right below Larnaca was a fresh-water lake; from Ayia Napa" - today a resort in the far south-east of Cyprus famous for its raves - "begins the western edge of the Atlantis plain, which goes all the way to the coast of Syria. The acropolis of the lost city is situated exactly seven miles off Cyprus." And the buildings that still stand on it, Mr Sarmast assures us, will be the oldest buildings the eye of modern man has ever fallen on, and will make "the pyramids of ancient Egypt look like modern buildings" in contrast.

Unlike the Parthenon in Athens, which succumbed slowly to dust, pollution and the attrition of the centuries, Atlantis was drowned at a stroke by a mighty, god-sent tidal wave with the extraordinary result, Mr Sarmast is sure, (and he has his bathymetric maps to back him up) that Atlantis is still there.

If Plato is to be believed, the buildings of Atlantis were dazzling in their splendour: the innermost temple (where the god Poseidon, incidentally, fathered Atlas on the mortal female Cleito) was vast, its walls covered with silver, the interior clad with ivory, decorated with gold, silver and orichalcum. It was filled with golden statues, the most magnificent being of Poseidon in a chariot drawn by winged horses, surrounded by 100 Nereids riding on dolphins.

Mr Sarmast claims all these marvels are still in place. "The city of Atlantis is submerged under thousands of feet of water," says Mr Sarmast, "a situation that has fortunately insured the preservation of the colossal ruins. The ultimate aim is to locate and film its many stone temples, palaces, roads, bridges and artefacts. The whole world is going to shift to this island," he predicts. "It will be the greatest archaeological discovery in history. It will change religion, it will change politics, science. The ramifications are almost endless. Cyprus will be the talk of the world for the next 500 years."

Why should Mr Sarmast, 39 years old and with no qualifications in archaeology or ancient history, persuade a major television company that he is right when so many other seekers have been completely wrong? He's got several things on his side. He is staking his claim in the age of the internet, a medium which incubates New Age fantasies the way mould grows on a compost heap. He is at work in an age with a vast appetite for vivid speculative documentaries, an age also when scientists are so desperate for a crust that they think nothing of lending their names to a cause as laughable as Intelligent Design (though to be fair, the involvement in Mr Sarmast's project to date of specialists from the National Oceanic and Atmospheric Administration has been limited to providing 2,500 pages of seafloor maps).

But the main thing that marks him out is certainty. He does not have a theory; he has made "an unprecedented series of findings." He is not hopeful of success: he is already triumphant. Of the latest ocean floor images "published today to the world press and scientific community on the official Cyprus Atlantis Expedition website," his spokesperson declared earlier this month, "triumphant expedition leader American Robert Sarmast is confident [the images], which include a 3km-long straight wall intersected at right angles by another wall, will finally silence any remaining scepticism about his long-standing claims that modern Cyprus is what remains of a much larger and now partly sunken landmass - a landmass which fits Plato's description of the ancient land of Atlantis perfectly."

"Robert Sarmast gave up a promising career in architecture to pursue his lifelong passion for ancient history, world mythology and the search for lost civilisations," notes his website. The first fruits of that passion, his "breakout book", was Discovery of Atlantis: the Startling Case for the Island of Cyprus, published in 2003. Last year he arrived in the island and launched his first expedition, and he has been coming back regularly since.

Inevitably there are people who would deny him glory, who point out, in contrast to the island described by Plato, Cyprus is neither beyond the Pillars of Hercules, nor in an ocean, nor bigger than Libya and Asia combined. And there are others with even keener objections.

Last October a French geologist called Dr Michel Morisseau, who lives in Cyprus, challenged Mr Sarmast to a public debate. "I was shocked by the news [of his claim to have identified the acropolis]," he said.

"Because it has nothing to do with the geological facts ... How can you prove that a mythical city, supposedly built above sea level, is now sitting 1,800 metres below sea level without any damage? What was the process of the subsidence? If there was a rapid subsidence it should be upside down and you would not be able to recognise anything. You would not be able to recognise a wall ... Everything would have been destroyed."

A few days later a German physical geographer and marine geologist, Ulf Erlingsson, logged his own criticisms from his home in Florida. Mr Sarmast's Atlantis, he pointed out, "is not positioned outside the Pillars of Hercules, nor in an ocean." Mr Sarmast has not stumbled on the walls of the temples on Atlantis's acropolis, he said, but an underwater volcano. By chance Dr Erlingsson and two other scientists surveyed the area in 2003. "The real killer of the hypothesis," he went on, "is that 100,000-year-old mud volcanoes exist on the spot. How then could it have been dry land only 12,000 years ago?"

Mr Sarmast has yet to rebut these objections in detail. But maybe it doesn't matter. He has yet to break his stride. People love a good yarn, particularl y with the lure of solid gold, dolphin-riding Nereids gleaming at the far end. And even if there is no gold off the Cyprus coast, there seems to be plenty of money in the venture - earlier this year the Cyprus Tourist Organisation announced it was renewing its funding, to thwart a feared counter-bid from the Turkish Cypriot authorities.

There is no good reason to believe that Atlantis really existed, any more than Plato's famous cave existed: like the cave, it was probably just the philosopher's vivid way of getting across a moral message.

The first person to pour scorn on the Atlantis story was Aristotle. " The man who dreamed it up," he said, speaking of Plato, "made it vanish." Yet no matter how many wise people say it's all tosh, many will watch Mr Sarmast's progress agog, eager to believe there is something real down there, not just a muddy old volcano.

Locating the lost city

HELIKE

Near the Gulf of Corinth, the ancient city of Helike fits the Atlantis profile as it was a flourishing city struck down in its prime by an earthquake in 373BC. The city state was the centre of a cult of Poseidon, second only in importance to the Oracle at Delphi. Generations of fishermen in the Gulf have told of snagging their nets on statues of, an apparently wrathful, Poseidon. BBC Horizon claimed to have located the site.

(Plausibility 8)

SANTORINI

Considered by many as the likeliest because Plato's description of a grand civilisation matches what we know of the Minoans whose rule stretched from Crete to the volcanic island of Santorini. Nay-sayers point out that the dates and scale of Plato's story don't match what we know of the violent seismic past of Santorini. However, if Solon, the source of Plato's writings, exaggerated the extent of Atlantis we could be in business - which is what the numerous Hotel Atlantises are on the modern-day holiday island.

(Plausibility 7)

TROY

As if the mystery of Atlantis location weren't enough, the author and geologist Bernhard Zangger has bound it up with the hunt for Troy.

(Plausibility 5)

CORNWALL

The Moscow Institute of Meta-History has avoided the obvious in locating Atlantis about 100 miles off Land's End. The site, at the edge of the Celtic shelf which may have been dry before the Ice Age, is thought locally to be the site of the competing myth of the City of Lions.

(Plausibility 2)

BOLIVIA

If anyone is going to find Atlantis then surely it should be someone with a name like Colonel John Blashford-Snell. Unfortunately, the soldier-explorer has stretched credibility by claiming that satellite images of a site in Bolivia fit Plato's description. "A lot of people laugh at us," said the colonel.

(Plausibility 1)

SOUTH-EAST ASIA

A Brazilian nuclear physicist, Nuñes dos Santos, has told us we've been looking in the wrong places for 30 years. We should be looking in the Indo-Pacific, he insists. This summer a team of well-funded Malaysians deploying remote-sensing satellites will try to prove him right.

(Plausibility 4)

SARDINIA

Sergio Frau, an Italian writer, starts by telling us the Pillars of Hercules are not in Gibraltar but in the SicilianChannel. So Atlantis was really Sardinia. He says its inhabitants were hit by an earthquake and migrated to the mainland to form the basis of what became Roman civilisation.

(Plausibility 3)

SRI LANKA/INDIA

South Asian Atlantis-hunters point out its similarities to stories about the submerged Kumari continent, between Sri Lanka and India.

(Plausibility 2)

FINLAND

A Finnish amateur historian, Ior Block, tells us the lost city is in southern Finland where a community lived in the Ice Age. Inevitably this theory is part of a grander saga of oral history passed down through generations of Blocks dating back to the creation of language itself.

(Plausibility 1)

IRELAND

Combine a Swedish oceanographer and a book called Mapping Fairy Land and what do you get? The revelation that Atlantis was off Ireland.

(Plausibility 2)

CARIBBEAN

The idea that Atlantis is really a submerged island off central America is based on the musings of a Canadian-Hungarian geologist-topographer who called his book Atlantis: The Seven Seals.

(Plausibility 1)