The Antikythera mechanism, discovered in 1901 aboard a shipwreck near the island of Antikythera, was dated to come from two centuries B.C. It is one of the most investigated relics, as its technology seems to be far too advanced for the period it came from. It appears to be some type of analog computer, capable of calculating the position of the Sun, Moon and some five planets relative to Earth.
Many people have been studying them, sometimes for just personal curiosity, or professional skill. One such person is the guy behind this YouTube video. It was a months long project involving tools not everyone has access to, and it shows clearly just how complex the mechanism really is.
Oddly enough it also seems to include a "Games calculator", indicating when the Olympic games were being held. All other stuff was navigational and astronomical in nature.