An ancient piece of clockwork found at the bottom of the sea. Part of some vastly complicated brass machine? Built by the Ancient Greeks? Does it track the moements of planets? Is it part of one of Archemede's fabled observatories?
Well, it's a part of the ship's coffee-machine ofcourse!
On a serious note: since it's mechanical, the device can't be that spectacular. Most mechanics from those times tended to be simple aids for applied mathematical calculations. I believe the article
If I recall correctly, from one of my old VonDanniken books, its a computational device for keeping track of the stars and their alignments and all that astrological stuff.
Two webpages (by my dad, no less) explaining how it works. It's a differential gear, and it's pretty sophisticated for the level of technology they were supposed to have.