The Antikythera Mechanism

What you are looking at is a kind of astrolabe, and this one is known as the Antikythera Mechanism. It was found in a very well preserved shipwreck in the Mediterranean Sea, and it was very likely something similar to this:

Without that added bit of context, it would be difficult to imagine the ruined version to be similar to the actual astrolabes used during that particular era. This particular paper (pdf alert) is essential to understanding why the find was so important.