They found lots of statues, jewelry and coins, and a strange lump of corroded metal.
Examination revealed that the "rock" was a heavily encrusted and corroded machine.
It had survived the shipwreck in three main parts and dozens of smaller fragments.
The device itself was surprisingly thin, about 33 cm (13in) high, 17 cm (6.75in) wide and 9 cm (3.5in) thick, made of bronze and originally mounted in a wooden frame.
Scans of the mechanism in 2008 found that it may also have been used to predict eclipses, and record important events in the Greek calendar, such as the Olympic Games.
The scans also revealed the mechanism was originally housed in a rectangular wooden frame with two doors, covered in instructions for its use.
On the back were two further dials displaying information about lunar cycles and eclipses. The calculator would have been driven by a hand crank.