The Antikythera mechanism is a special computer composed of scores of bronze cog wheels that charts the positions of the sun, moon, and the five visible planets relative to earth. It was made about 80 BC, was on board a ship apparently carrying plunder from the east to Rome (possibly after the 3rd Mithridatic War) and lay on the seabed for over 1900 years, during which time it became heavily corroded and damaged. The wreck was discovered near the tiny Mediterranean island of Antikythera, which gave its name to the object. Pieces recovered by deep-water divers at the beginning of the twentieth century are still being studied (another 75 or so small fragments turned up in the National Archaeological Museum in Athens in 2005; the total number of fragments is now 82), and it is obvious that the last word on this device has yet to be written. However, it was already clear by 1970 that this machine was far more advanced than any described in detail in the surviving literary sources, including treatises on mechanics. Sophisticated gearing such as this could have been a feature of many precision instruments in antiquity. Gearing for heavy applications, such as to drive mills by waterwheel, used wooden components, not bronze.
An early article on the Antikythera Mechanism by D. de Solla Price was published in Scientific American June (1959) 60-7. A much more recent paper by R Rice from 1995 is here. Another recent discussion (1998) by C Zeeman is here. Another, albeit brief, discussion, by M Wright of the Science Museum, who has built another reconstruction, is here (thanks to David Harvey for first alerting me to this), and a preprint of a paper he delivered at the 2nd International Conference on Ancient Greek Technology in Athens, October 2005, and published in the Proceedings in 2006, is here. The Wikipedia entry, which summarizes the history of the study of this object and appears to have been written by someone connected with the AMRP (Antikythera Mechanism Research Project ), is here. More pictures of the pieces in the Archaeological Museum in Athens are here (under 'Bronzes'). The Antikythera Mechanism Research Project, with the latest information, amazing photos, and ongoing discussion forum, is here.
Only part of the mechanism survives. Some idea of how corrosion affected objects on the Antikythera wreck can be seen from the following photo of one of the marble statues that was on board this booty-laden boat ('the Romans took great pains to make all the finest things of Greece adornments of Rome', Prokopios Wars 8. 21. 14). The left side has been almost completely eaten away. There is no better illustration of the fragility and ephemeral nature of beauty.
Perhaps an anonymous poem in the Greek Anthology (9. 822) refers to the Antikythera mechanism, or something like it.