In 1900, a team of sponge divers submerged themselves into the sea off the island of Antikythera between mainland Greece and Crete and emerged with a curious find. Exploring a Roman shipwreck, they brought up a bronze-and-wood object that defied description. It looked like a clock, but not quite; it resembled a gear or wheel, but held no hints that it was once attached to any mode of transport.

The Antikythera mechanism, as it came to be known, was largely ignored for the next half-century, as researchers were preoccupied with the other artifacts found in the shipwreck and lacked the tools to see through its corroded exterior. But in recent decades, the importance and capability of the Greek-born device thought to date to the 1st or 2nd century BCE has slowly been unspooling. Depending on how you define the term, it may be the world’s first computer.

Although investigation into the strange box began when it was first retrieved, it wasn’t until researchers began using radiographs to examine its inner workings that the true nature of the Antikythera mechanism was discovered. Inside, they found 30 bronze gears that was operated by a hand crank. About the size of a shoebox, it acted as an astronomical calendar, predicting the cycles of the solar system in the coming decades. Lunar months and eclipses could also be anticipated. Greek zodiac signs and Egyptian calendar dates appear on the front. By turning the dial to one of the 365 days on its face, the user could anticipate the exact position of the Sun and Moon.

While X-rays could provide some basic structural information to investigators, microfocus X-rays, originally developed to find tiny fractures in turbine blades, were also put to use, revealing faded inscriptions that haven’t been visible for thousands of years. Tiny letters 1.2 millimeters tall told users what they might see when operating it.

There’s still much left to learn about the Antikythera mechanism. No one is quite sure who made it or for what purpose, although it’s possible a school may have been the beneficiary of its results. It’s also possible the Antikythera mechanism was devised to tell fortunes, as it provided information about eclipses that were associated with good and bad omens.

The remains of the Antikythera mechanism are housed in the National Archaeological Museum in Athens, Greece, along with sculptures and other relics from the dive. With the site of the wreck still being scouted, it’s possible more answers about this strange, impeccably designed machine may still be lurking at the bottom of the sea.