The Antikythera mechanism
A lump of corroded bronze from a Roman-era wreck turned out to hold a gear train with no known ancestor for a thousand years.
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The Antikythera Wheels and the Thousand-Year Gap
The Antikythera mechanism is a geared bronze calculator recovered from a shipwreck off the Greek island of Antikythera in 1901, made between the late third and early first centuries BC. Its thirty surviving gears predicted eclipses and modelled the Moon's uneven motion. Its own reconstructors call it a creation of genius. Here is what it does, how it was read, and why nothing like it survives for so long afterward.