A 2,100-year-old "computer" found in a Roman shipwreck may have acted as a calendar for the Olympic Games, scientists report in Nature journal.
The Antikythera Mechanism has puzzled experts since its discovery by Greek sponge divers in 1901.
Researchers have long suspected the ancient clockwork device was used to display astronomical cycles.
A team has now found that one of the dials records the dates of the ancient Olympiad.
This could have been to provide a benchmark for the passage of time.
The device is made up of bronze gearwheels and dials, and scientists know of nothing like it until at least 1,000 years later.
Tony Freeth, a member of the Antikythera Mechanism Research Project, said he was "astonished" at the discovery.
"The Olympiad cycle was a very simple, four-year cycle and you don't need a sophisticated instrument like this to calculate it. It took us by huge surprise when we saw this.
"But the Games were of such cultural and social importance that it's not unnatural to have it in the Mechanism."
The technique of X-ray computed tomography gave the researchers a 3D view of its 29 surviving gears. High-resolution imaging provided them with a close-up of tiny letters engraved on the surface.
The device's "subsidiary dial" was once thought to be a 76-year "callippic" calendar.
However, Mr Freeth and his colleagues have now been able to establish from its inscriptions that it displays the 4-year Olympiad cycle.
Instead of one Olympics as there is today, the ancient Olympiads, called the Panhellenic Games, comprised four games spread over four years.
The four sectors of the dial are inscribed with a year number and two Panhellenic Games: the "crown" games of Isthmia, Olympia, Nemea and Pythia; and two lesser games: Naa (held at Dodona) and a second game which has not yet been deciphered.
In addition, the team was able to identify the names of all 12 months, which belong to the Corinthian family of months.
Corinth, in central Greece, established colonies in north-western Greece, Corfu and Sicily, where Archimedes was established.
Archimedes, whose list of exploits included an explanation for the displacement of water and a screw pump that bears his name today, died there in 212 BC.
The Antikythera Mechanism was "almost certainly made many decades" after his death, according to Alexander Jones, a professor at the Institute for the Study of the Ancient World in New York, US.
If it came from Syracuse, the dial could have been made by the school of scientists and instrument-makers he inspired.
The priceless artefact was found by a sponge diver amid other treasures on a wreck near the tiny island of Antikythera between Crete and the mainland. It is on display at the National Archaeological Museum in Athens.