Antikythera Mechanism

A bronze computer from the wrong millennium, pulled from the sea floor.

Pulled from a Roman shipwreck in 1901 and ignored in a drawer for fifty years, until X-rays found thirty bronze gears computing lunar anomalies with differential gearing — technology otherwise absent for fourteen centuries. The mechanism is singular: no precursor, no successor, no workshop tradition. Either the ancient world's engineering literature is almost entirely lost, or someone built one perfect machine. Both options should bother you.