On congested, dusty Alexandras Avenue, the secrets of the sea seem a world away. But in Room 625, on the sixth floor of Greece's gargantuan police headquarters, the watery world of ancient shipwrecks and other archeological riches occupies the attention of Giorgos Gligoris.

The veteran officer oversees the Hellenic Police Force's antitrafficking unit, battling smugglers bent on snatching treasures from the seabed.

Traffickers have caught on to the fact that there are more than 12,000 shipwrecks in Greek waters. Many of the submerged gems date back to the Golden Age of the fifth century BC. Armed with archeological service maps acquired on the black market, burgeoning numbers of international smugglers have made it their mission to locate the wrecks, authorities say.

''In the United States and Europe, ancient Greek artifacts are, sadly, very fashionable," Gligoris said. ''Unfortunately, nouveaux riches like them because they're not only pretty and look good in their sitting rooms, but happen also to be a great investment."

Gligoris said some looters are coming to Greece and posing as wealthy tourists on yachts. ''They arrive, supposedly on a cruise, when their real intention is to locate wrecks and whisk gold and bronze antiquities out of the country," he said.

Criminal gangs, emboldened by the explosion of Internet auction houses, have come to see the acquisition of antiquities as a way to launder ill-gotten gains.

Faced by rising threat of piracy, Greece's center-right government has made locating and protecting historic wrecks a top priority. In the past five years, state-employed underwater archeologists have discovered 30 ancient wrecks -- compared with five wrecks in the decade before that -- at depths of up to about 1,970 feet, dispatching coast guard officials to protect the finds.

But the advances of technology, not least the ready availability of powerful search equipment, often mean that the modern pillager gets to the vessels before the country's overworked archeologists.

While high-tech wizardry has helped academics better understand the boundaries of deep-sea archeology, it has also allowed amateur treasure hunters to illicitly tap into Greece's vast underwater heritage. Increasingly, looters can afford to buy the sophisticated sonar equipment needed to locate potential treasure troves on land and sea.

''Technology has no principles," said Katerina Delaporta, who heads the Department of Marine Antiquities at the Greek Ministry of Culture. ''Looting has become a big danger because the development of diving techniques, and equipment is being used very effectively by people to plunder undersea archeological sites."

With shipwrecks scattered around the Aegean and Mediterranean seas, patrolling them is practically impossible, Gligoris said.

Once looters bring the artifacts to the surface, authorities have a difficult time proving that the items have been stolen without previous photographic or archival evidence of their existence, he explained.

''Greece has the longest coastline in Europe. The Mediterranean is a very big place," Gligoris said. ''We would need millions of archeologists and divers to police these waters, and the fact of the matter is there are only 15 of us who work in the country's antitrafficking department."

Thanks to the Romans's penchant for original classical and Hellenistic statues, thousands of sculptures are believed to have been spirited out of Greece by Roman invaders. Specialists also believe the Aegean seabed is littered with masterpieces that went missing in storms.

Many of these priceless pieces are thought to have ended up in the hands of antiquities smugglers after fishermen accidentally netted them. Invariably, the works are whisked out of Greece in fruit and vegetable trucks, according to police who have successfully stopped many such vehicles at frontier checkpoints.

Once trafficked, antiquities can change hands as many as five times before ultimately reaching the display room of an auction house or museum.

''It's not just this new breed of looter. The fisherman's trawler, also, has been the curse of underwater archeology for the past 200 years," said Harry Tzalas, a leading maritime expert and a specialist in the reconstruction of ancient ships. ''Evidently, there is a market out there, and the way we should deal with the problem is not with diver-policemen but by offering rewards that make it attractive for fishermen to hand over their finds as soon as they are discovered."

In the past seven years, four masterpieces, including a statue of the Roman emperor Octavius, have been delivered to authorities by fishermen in return for rewards, Ministry of Culture records show.