The Antikythera mechanism: someone built an analog computer 2,000 years ago and we only figured it out with X-rays

Fished out of a shipwreck in 1901, looked like a corroded lump for fifty years. Turns out it's a geared machine that predicted eclipses and tracked planetary positions — gear-cutting precision Europe wouldn't see again for over a millennium.

https://en.wikipedia.org/wiki/Antikythera_mechanism

My favorite detail: it had a dial for the four-year Olympiad cycle. They built a sports calendar into their eclipse computer.