The Antikythera mechanism

This object, and some other fragments found in the early 1900s off the Greek island of the same name, is a complete anachronism. Dated to 150-100 BCE nothing similar is known till a thousand years later. The mechanism seems to have been made to calculate astronomical events and positions and is of remarkable complexity for its age.

Results of a new X-ray study^[1] suggest that in addition the Antikythera mechanism also displayed the timing of the original Olympic Games.
You need a subscription to Nature to read the full article but there is a great video available here. It's worth watching.

EDIT: it seems you may not need a subscription to read the article. The video is still good to see.
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1. Tony Freeth, Alexander Jones, John M. Steele & Yanis Bitsakis 'Calendars with Olympiad display and eclipse prediction on the Antikythera Mechanism' Nature 454, 614-617 (31 July 2008)