the Antikythera mechanism
The mechanism is the oldest known surviving geared mechanism, made from bronze in a wooden frame, and has puzzled and intrigued historians of science and technology since its discovery. The most commonly accepted theory of its function is that it was an analog computer designed to track the movements of heavenly objects.
Recent working reconstructions of the device support this analysis. The device is all the more impressive for its use of a differential gear, which was previously believed to have been invented in the 16th century AD.
2006 article: close to revealing a puzzle concerning astronomical and mathematical knowledge