When it was first proposed, it seemed like a good idea: open up the Greek seas to divers and create a paradise for tourists underwater. Those who backed the law never thought of it as a windfall for looters, nor did it occur to them that it might put the acquisition policies of museums under further scrutiny.

But the Greek parliament's unprecedented step last month to allow divers access to the once forbidden coastline has raised fears that archaeological riches preserved in an untouched world will be taken by ruthless thieves.

"There are treasures in our seas," says Dimitris Athanasoulis, president of the Archaeologists' Association. "This will open the floodgates to smugglers. It'll serve to encourage them at a time when evidence shows the trafficking of antiquities is on the rise."

Last month, as Athens announced legal action against California's Getty Museum to reclaim an array of antiquities whose rightful owners, according to authorities, died at least 2,000 years ago, the row reached a new pitch. At issue are thousands of shipwrecks believed to be buried in the Mediterranean. Greece is thought to host most of these submerged gems, with an undisclosed number, say experts, dating to the golden age of the 5th century BC. And, like later vessels from the Roman, Byzantine and early modern periods, those ships sank with priceless cargoes intact.

"If you think of at least one ship going down a year then there would be at least 6,000 of them down there now," says Katerina Delaporta, who heads the department of marine antiquity at the ministry of culture. "There could be double that," she says. "What is really bad is that this legislation not only contradicts constitutional laws that go back to the foundation of the Greek state on how our archaeology should be protected, but it also allows people to dive at great depths with the latest technology."

Previously, divers were given access to just 620 miles of the 10,000 miles of Greek coastline. Under the new legislation, however, they will be able to explore vessels and "archaeological parks" along the entire seabed freely. Until now, Greek authorities have gone out of their way to locate and protect historic wrecks. In the last five years, state-employed underwater archaeologists have found 35 ancient ships - compared with five in the decade before that - at depths of up to 600 metres. In total, a thousand have been catalogued.

But technological advances often mean modern pillagers get to such jewels before overworked archaeologists. While hi-tech tools have helped specialists better understand the boundaries of marine archaeology, they have also allowed amateur treasure-hunters to tap into Greece's vast underwater heritage, says Dr Delaporta.

"The sea is not like a museum. It can't be guarded round the clock and unfortunately technology has no principles," she says. "Looting is a big danger." It is not only wrecks that are attractive to looters. The Aegean is also thought to be littered with masterpieces lost in storms, thanks to Roman invaders' penchant for classical and Hellenistic statues. Since 1997 four statues, including a magnificent rendition of Roman emperor Octavius, have been delivered by fishermen to the state in the hope of rewards.

The problem of undersea plundering is part of the much wider problem of looting. Net profits from the global trade in antiquities are now on a par with those from smuggling humans and drugs, according to culture ministry officials in Greece. Emboldened by the explosion of internet auction houses, an increasing number of looters, they say, are linking up with criminal gangs seeking to launder ill-gotten gains through the international art market.

Nobody knows this better than Giorgos Gligoris. As head of the police squad set up to combat antiquities trafficking, the detective frequently dons the sharp suit of a collector to infiltrate art smuggling circles. From his cramped sixth-floor office in Athens's gargantuan police headquarters, he explains that, Europe-wide, the "bad economic climate" has spurred a proliferation of looters. This year alone, his 12-member team has seen a huge rise in valuable Byzantine icons being filched from monasteries.

"Put simply, profits are phenomenal and looters are running riot," he bristles, cigarette ash flying as he raises his hands in despair. "In the US and Europe ancient Greek artefacts are, sadly, very fashionable. Nouveau riche like them because they're not only pretty and look good in their sitting-rooms, but they also happen to be a great investment." For Greece there is the added problem of the country being "like a museum without any guards or doors".

"And now Europe is border-free and there are far fewer checkpoints, it's even easier for traffickers," he sighs.

Experts believe that often antiquities are whisked out of Greece in fruit and vegetable trucks. Destination reached, they can change hands up to five times before arriving in the display room of an auction house or museum.

The apparent ease with which smugglers have learned to move has given birth to a new type of menace in the form of looters posing as tourists, Mr Gligoris says. Last summer his unit stopped a yacht in broad daylight brimming with valuable amphorae, or jars, as it pulled out of port on the remote island of Mytilene.

In 2004, he says, his department confiscated 1,401 ancient artefacts in 24 raids throughout the country. "A lot of smugglers are coming here posing as wealthy tourists on yachts," he explains. "They arrive, supposedly on a cruise, when their real intention is to locate wrecks and whisk gold and bronze antiquities out of the country."

The work of anti-traffickers is made harder because of the near-impossibility of being able to prove that an artefact is stolen without previous photographic or archival evidence of its existence, he says. Last year, Greek officials discovered this to their cost when 17,000 stolen antiquities - enough to stock 10 museums - were found in the collection of the disgraced British art dealer Robin Symes. The unearthing of the hoard, the decision to take legal action against the Getty - after years of abortive diplomacy - and the dispute over allowing diving have all reinvigorated the campaign to stop Greece's heritage being spirited away.

For Mr Gligoris, there are times when he feels he is winning. "We do have moments of light when we catch smugglers. And when that happens, there's a lot of dancing and singing in this office."

Case study

'He was counting the money'

Two years ago officer Giorgos Gligoris was tipped off about a "hot sale" in the province of Epiros. A retired civil servant conducting illegal excavations outside Ioannina had unearthed two 4th century BC statues - an Aphrodite and a gorgeous youth. He had stashed them in the basement of his block of flats and was looking for a buyer. He was prepared to sell them for £280,000. The detective filled his briefcase with cash and drove up to the north-western city. Members of his team would shadow him but until the mission was over he would use his alter ego: a Greek gallery owner who lived and worked in Switzerland. "We agreed to meet in a cafe and the trafficker took me to the building in question to show off the pieces," Mr Gligoris said. "They were beautiful, and I instantly said I would buy them but suggested have lunch first." When they got to the restaurant the detective alerted his team. "The old man was counting the money when they nabbed him. It was brilliant," said Mr Gligoris. "It took us two years to win the trust of the men who tipped us off and we were lucky because it wasn't dangerous. Other times it's been hairy. One guy, a trafficker in the army, kept his fingers on a grenade in his pocket the entire time we were negotiating. You can't show fear because if you do, you've lost the game."