A corroded mechanism recovered by sponge divers from a sunken wreck near the Greek island of Antikythera in 1902 changed the study of the ancient world forever.

The Antikythera Mechanism, a system of bronze gears from the 2nd century BC, was used to calculate the date of the Olympic Games based on the summer solstice.

Its mechanical complexity was unequaled for 1,000 years, until the cathedral clocks of the Middle Ages.

Archaeologists believe hundreds more wrecks beneath the eastern Mediterranean may contain treasures, but a new law opening Greece's coastline to scuba diving has experts worried that priceless artefacts could disappear into the hands of treasure hunters.

"The future of archaeology in this part of the world is in the sea," said marine archaeologist Harry Tzalas.

"This law is very dangerous, it opens the way to the looting of antiquities from the seabed, which we don't even know exist."

Greece's 1932 antiquities law says all artifacts on land and in the sea belong to the state, but it does not regulate scuba diving, developed in the 1940s by Frenchman Jacques Cousteau.

A new law implemented in 2007 and designed to promote tourism opens most of Greece's 15,000km coastline to scuba divers, except for about 100 known archaeological sites.

Greece's archaeologists' union and two ecological societies have appealed for the law to be rescinded.

Meanwhile, some tour companies are luring tourists with the promise of ancient artifacts.

"Scuba diving in Greece is permitted everywhere ... Ideal for today's treasure hunter," says the website scuba-greece.com.

The director of antiquities at the Culture Ministry, Katerina Dellaporta, says metal detectors and bathyspheres allow treasure hunters to find artifacts with ease in the Adriatic and Aegean.

"It's good to have tourism, but we must protect antiquities," she said.

"Not every diver is an illegal trafficker... but we need to ensure these treasures remain for future generations."

A vast museum

Most of the world-famous bronzes in Greece's National Archaeological Museum, such as the 5th-century BC statue of Poseidon hurling his trident found off Cape Artemision, were salvaged from the sea.

Statues on land tended to be destroyed or melted down for coins or weapons.

Some were found in shallow-water shipwrecks like the one off Antikythera, believed to be a 1st century BC Roman ship carrying a haul of ancient Greek art back to Italy.

Other precious statues were dredged from the deep ocean in fishermen's nets.

Greece offers handsome rewards to prevent relics falling into private hands.

It paid 440,000 euros ($872,000) to a fisherman for a female torso off the island of Kalymnos in 2005.

"The sea is a vast museum of shipwrecks... that is rewriting history as we know it," said Shelley Wachsmann, professor of marine archaeology at Texas A and M university, who opposes the law.

"The risk is that Greece will become like Italy, where there is nothing left above 70 meters [underwater]," he said.

Divers can ruin an excavation by taking mundane items such as amphorae that shed light on everyday life, archaeologists say.

In deeper waters, the main threat to antiquities is trawler fishing, which disturbs sites and damages artifacts.

Archaeologists know of many treasures still lost at sea.

About 5,000 pieces from the collection of Luigi Palma di Cesnola - who helped found New York's Metropolitan Museum of Art - disappeared in a Mediterranean shipwreck in the 1870s.

For Tzalas, the "holy grail" is the lost city of Helike, which disappeared under the sea off western Greece one night during an earthquake in 373 BC.

A wreck from Crete's Minoan civilization would also provide a first glimpse of the Bronze Age culture's maritime activities.

Many wrecks already discovered, including sites off the Aegean island of Kalymnos, have not been excavated because of lack of funds, leaving them prey to looters.

"It's not fair to tell the Greeks they have to use up their money protecting this, when it's patrimony of the entire world," said Mr Wachsmann.

"This is an international responsibility."