...choose a mystery:
Ancient Aeroplanes? - South American model aircraft from thousands of years ago?
The Baghdad Battery - Ancient device that produces a 2 volt electrical current.
Aerial Maps - Mediaeval maps showing Antarctica, the Americas and more, that were themselves, copied from even OLDER maps.
Antikythera Mechanism - An accurate mechanical model of the solar system from the Ancient Greeks.
The Iron Pillar of New Delhi - Hundreds of years old yet has refused to rust.