Who invented the first mechanical computer?

The first mechanical computing machine was invented in Greece circa 100 BC: It was used for navigation. it was called the Antikythera mechanism.
It is remains unknown as to the origins and dating of the first mechanical computers. For example by definition the Abacus is mechanical and can perform computations. It has been around and remains used for the past thousands of years. The Abacus and the archeological discovery of the Greek computer remain with unknown inventors.