From Murray’s Human Accomplishment:
The problem with the standard archaeological account of human accomplishment from [the ancient world] is not that the picture is incomplete (which is inevitable), but that the data available to us leave so many puzzles.
The Antikythera Mechanism is a case in point… The Antikythera Mechanism is a bronze device about the size of a brick. It was recovered in 1901 from the wreck of a trading vessel that had sunk near the southern tip of Greece sometime around –65. Upon examination, archaeologists were startled to discover imprints of gears in the corroded metal. So began a half-century of speculation about what purpose the device might have served.
Finally, in 1959, science historian Derek de Solla Price figured it out: the Antikythera Mechanism was a mechanical device for calculating the positions of the sun and moon. A few years later, improvements in archaeological technology led to gamma radiographs of the Mechanism, revealing 22 gears in four layers, capable of simulating several major solar and lunar cycles, including the 19-year Metonic cycle that brings the phases of the moon back to the same calendar date. What made this latter feat especially astonishing was not just that the Mechanism could reproduce the 235 lunations in the Metonic cycle, but that it used a differential gear to do so. Until then, it was thought that the differential gear had been invented in 1575.
See also Wikipedia.