Antikythera mechanism
When the mechanism was found, it consisted of corroded metal plates. Before x-rays could be used to show what was "inside", it was very difficult to judge what the mechanism did. Many of the parts that make up the mechanism were lost either during the roughly 2000 years it had laid on the sea floor, or shortly after discovery. Many gears have not "survived" in one piece. None of the scales is complete, and only one indicator survived as a fragment. The Antikythera Research project has produced a reconstruction; it added seven gears that were probably part of the device. There are 35 gears, and seven displays. There are many hints that the device was also used to calculate and show the movement of stars, even though none of the necessary gears or displays have been found.