They were hopelessly outnumbered, but even then the Greeks knew it would be the battle that could change history.

The Asian invaders had entered the Aegean. The "comeliest of boys" had been castrated; the throats of the "goodliest" soldiers ripped out.

Mounted on his marble throne, Xerxes, Persia's formidable warrior king, looked over the bay of Salamis, confident that he was about to enslave Europe. But instead of victory came defeat.

As the Greeks' triremes trapped the Asian fleet, smashing it with their bronze rams, Xerxes watched incredulously. His soldiers, he said, were fighting like women.

That was 480BC. Nearly 2,500 years later, the quest to better understand the battles that the victorious Greeks would see as a defining point in their history has reached new heights, as experts yesterday began searching for the lost fleets of the campaign in the northern Aegean.

In the world of underwater archaeology the hunt for the legendary armadas is the expedition that might, just, scoop all others.

Topping the international team's wishlist is the remains of a trireme, the pre-eminent warship of the classical age.

"This is high-risk archaeology," said Shelly Wachsmann of Texas A&M University and the team's co-leader. "Discovering a trireme is one of the holy grails. Not one has ever been found."

The Persians' defeat at Salamis is seen as one of the first victories of democracy over tyranny, a crucial moment in western history.

Without it, say scholars, there would have been no Golden Age and the world would have been a very different place.

All of which makes this week-long mission more poignant as experts try to find out how the Greeks managed to defeat a much bigger and better-equipped enemy.

Although archaeologists have discovered ancient Greek and Persian ships, they have always been cargo vessels.

For their guide around three of the five sites where Persian and Greek vessels are believed to have sunk - the Magnesian coast of Thessaly, Artemision in northern Euboea and the "hollows of Euboea" - the scholars have Herodotus.

Known as the father of history, the 5th century BC historian chronicled the wars in his masterpiece, The Histories. But while his story is a good read, few artefacts have emerged to support it.

"This is a reversal of how we usually work in that we know the history but lack the physical evidence," said Katerina Delaporta, who heads Greece's department of underwater antiquities and is co-leading the project.

Previously, she said, the search would have been impossible because of the technical requirements involved. With the passage of time and the Aegean's unpredictable weather conditions, maritime experts believe the wrecks will be buried under mud and silt. That means surveying the seabed at depths of up to 600 metres where visibility is limited. Among the team's state-of-the art equipment are sonar scanners, a two-man submersible and a remote operated vehicle capable of sending video messages to the surface.

"This is the first time such sophisticated technology is being employed," she added.

More than 1,000 of the three-tiered triremes took part in the second Persian war.

But while ship sheds and dry docks have been unearthed, scholars have had to make do with images of the galley on pottery. The discovery of a trireme, either Greek or Persian, would not only unravel the mysteries of antiquity's greatest fighting vessel but shed light on the civilisation.

"Ships throughout time are among the most complex artefacts that any culture creates," Dr Wachsmann said.

Although the sea is more difficult to explore, it has the benefit of preserving artefacts better than if they were on land. Among the assembled geologists, archaeologists, historians and oceanographers there is no doubt that the ancient shipwrecks exist.

"It's just a question of finding them," said Stefanie Kennell, the director of the Canadian Archaeological Institute.

Because triremes had very little ballast, and when destroyed were unlikely to sink but float, archaeologists have long debated the likelihood of finding one. Most have set their hopes on finding a bronze ram, or the arms and armour that went down with the crews.

"If we can find part, or even the metal fittings of a trireme, it would add immeasurably to our knowledge of military seafaring in the early 5th century BC," Dr Kennell said.

In an earlier attempt to find the lost Persian fleet of the first Persian war, wrecked off Mount Athos in a storm in 492BC, the searchers discovered two helmets and a bronze-tipped spear butt.

But around Mount Athos, the waters are much deeper.

"Here, the chances of making more finds are higher," Ms Delaporta said.

The big prize - Salamis - has been left for now. But time is of the essence. With the technological advances a new kind of menace has arrived - looters, rushing to beat the scholars to the ancient wrecks.