Built in Greece around 150-100 BC and possibly linked to the astronomer and mathematician Hipparchos, its complexity was probably unrivalled for at least a thousand years, they say. "It's beautifully designed. Your jaw drops when you work out what they did and what they put into this," said astronomer Mike Edmunds of Cardiff University, Wales, in an interview with AFP.

"It implies the Greeks had great technical sophistication."

The Antikythera Mechanism is named after its place of discovery, where Greek divers, exploring a Roman shipwreck at a depth of 42 metres (136 feet) in 1901, came across 82 curious bronze fragments.

At first, these pieces, thickly encrusted and jammed together after lying more two millennia on the sea floor, lay forgotten. But a closer look showed them to be exquisitely made, hand-cut, toothed gearwheels.

It was clear that, within this find, 29 gearwheels fitted together, possibly making some sort of astronomical calendar. But of what, exactly?

For a quarter of a century, the textbook on the strange find was a work written by a historian of science and technology, Derek de Solla Price.

He hypothesised that the Mechanism in fact had 31 gearwheels, and did something pretty astonishing -- it linked the solar year with a 19-year cycle in the phases of the Moon. This is the so-called Metonic cycle, which takes the Moon 235 lunar months to the same phase on the same date in the year.

Edmunds' team, gathering experts from Britain, Greece and the United States, has now taken the tale several chapters forward.

In a paper published on Thursday in Nature, they describe how they used three-dimensional X-ray computation tomography and high-resolution surface imaging to peek beneath the Mechanism's surface, yet without damaging the priceless artefact.

There, they read inscriptions on the bronze cogs that had been unseen by human eye since that Roman ship came to grief aeons before.

The original device, they believe, is likely to have comprised 37 gear-wheels and comprised two clock-like faces, one front and one back, which would have fitted into a slim wooden box measuring 31.5 x 19 cm (12.5 x 7.5 cm) and a thickness of 10cms (four inches).

The machine was a 365-day calendar, which ingeniously factored in the leap year every four years.

And it not only provided the Metonic cycle, which was known to the Babylonians, it also gave the so-called Callippic cycle, which is four Metonic cycles minus one day and reconciles the solar year with the lunar calendar.

It could also predict lunar and solar eclipses under the Saros cycle, a 223-month repetitive interplay of the Sun, Earth and Moon. This function, presumably, would been useful for religious purposes, given that eclipses are traditionally taken as omens.

The Machine was also a star almanac, showing the times when the major stars and constellations of the Greek zodiac would rise or set and, speculatively, may also have shown the positions of the planets.

But even more impressive is a tiny pin-and-slot device that factors in a movement of the Moon that, for centuries, puzzled sky-watchers.

In this so-called main lunar anomaly, the Moon appears to move across the heavens at different speeds at different times -- the reason being its elliptical orbit around Earth.

"Newton used to say he would think about this until his head hurt," notes Edmunds, wryly.

This latter discovery prompts the scientists to wonder if the great Hipparchos, who drew up the first catalogue of the stars and wrote about the lunar anomaly in the 2nd century BC, may have had a hand in designing the Mechanism.

Adding circumstantial evidence to this theory is that the shipwreck was found to have jars and coins from Rhodes, where Hipparchos lived.

The computer is so advanced in its mathematics and technology that the history of ancient Greece may have to be rewritten, contends Edmunds.

"We now must ask: What else could they do? That's a difficult thing, because this is really the only surviving metallic artefact of its kind. Who knows what else may be lost?"

It was not until the end of the first millennium AD and the golden age of Islamic science that anything so technologically wondrous surfaced again, if the archaeological evidence is a guide.

This was an eight-geared astrolabe, depicting the movements of the Sun and Earth, by the Islamic astronomer al-Biruni in AD 996.

Had the Greeks' knowledge somehow survived and been transmitted across the centuries, to inspire al-Biruni? Or had it withered away and disappeared, leaving Islamic scholars with the task of rediscovering what had been known a thousand years before?

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