After 2,000 years under the sea, three flat, misshapen pieces of bronze at the National Archaeological Museum in Athens are all shades of green, from emerald to forest. From a distance, they look like rocks with patches of mold. Get closer, though, and the sight is stunning. Crammed inside, obscured by corrosion, are traces of technology that appear utterly modern: gears with neat triangular teeth (just like the inside of a clock) and a ring divided into degrees (like the protractor you used in school). Nothing else like this has ever been discovered from antiquity. Nothing as sophisticated, or even close, appears again for more than a thousand years.

For decades after divers retrieved these scraps from the Antikythera wreck from 1900 to 1901, scholars were unable to make sense of them. X-ray imaging in the 1970s and 1990s revealed that the device must have replicated the motions of the heavens. Holding it in your hands, you could track the paths of the Sun, Moon and planets with impressive accuracy. One investigator dubbed it “an ancient Greek computer.” But the X-ray images were difficult to interpret, so mainstream historians ignored the artifact even as it was championed by fringe writers such as Erich von Däniken, who claimed it came from an alien spaceship. It wasn’t until 2006 that the Antikythera mechanism captured broader attention. That year, Mike Edmunds of Cardiff University in Wales and his team published CT scans of the fragments, revealing more details of the inner workings, as well as hidden inscriptions—and triggering a burst of scholarly research.

The Antikythera mechanism was similar in size to a mantel clock, and bits of wood found on the fragments suggest it was housed in a wooden case. Like a clock, the case would’ve had a large circular face with rotating hands. There was a knob or handle on the side, for winding the mechanism forward or backward. And as the knob turned, trains of interlocking gearwheels drove at least seven hands at various speeds. Instead of hours and minutes, the hands displayed celestial time: one hand for the Sun, one for the Moon and one for each of the five planets visible to the naked eye—Mercury, Venus, Mars, Jupiter and Saturn. A rotating black and silver ball showed the phase of the Moon. Inscriptions explained which stars rose and set on any particular date. There were also two dial systems on the back of the case, each with a pin that followed its own spiral groove, like the needle on a record player. One of these dials was a calendar. The other showed the timing of lunar and solar eclipses.

Experts have been working to decipher inscriptions hidden inside the mechanism, in particular to understand the mechanism’s missing pieces, some destroyed, some probably still at the bottom of the sea. Though the pointers on the front face don’t survive, Alexander Jones, a historian at the Institute for the Study of the Ancient World in New York, says an inscription reveals that they carried colored balls: fiery red for Mars, gold for the Sun.

Also missing are the parts that drove the planetary pointers, leading to debate about exactly how they moved. Because planets orbit the Sun, when viewed from Earth they appear to wander back and forth in the sky. The Greeks explained this motion with “epicycles”: small circles superimposed on a larger orbit. According to Michael Wright, a former curator at London’s Science Museum who has studied the mechanism longer than anyone, it modeled epicycles with trains of small gears riding around larger ones. Though some experts have dismissed this as beyond the Greeks’ abilities, Jones says he will publish evidence supporting the idea later this year.

Other inscriptions hint at where the mechanism was made. Paul Iversen, a classicist at Case Western Reserve University in Cleveland, reports that the calendar includes month names used in Corinth and its colonies in northwest Greece. A dial that displayed the timing of major athletic festivals, including the Olympics, lists Naa, a festival held in northwest Greece, and Halieia, held to the south on the island of Rhodes. Perhaps the mechanism hailed from Rhodes and was being shipped north. The ancient philosopher Posidonius had a workshop in Rhodes that could have been the source; according to Cicero, Posidonius made a similar model of the heavens in the first century B.C.

The tradition of making such mechanisms could be much older. Cicero wrote of a bronze device made by Archimedes in the third century B.C. And James Evans, a historian of astronomy at the University of Puget Sound in Tacoma, Washington, thinks that the eclipse cycle represented is Babylonian in origin and begins in 205 B.C. Maybe it was Hipparchus, an astronomer in Rhodes around that time, who worked out the math behind the device. He is known for having blended the arithmetic-based predictions of Babylonians with geometric theories favored by the Greeks.

Regardless, the Antikythera mechanism proves that the ancient Greeks used complex arrangements of precisely cut wheels to represent the latest in scientific understanding. It’s also a window into how the Greeks saw their universe. They came to believe that nature worked according to predefined rules, like a machine—an approach that forms the basis of our modern scientific views. Edmunds argues that this “mechanical philosophy” must have developed as a two-way process. The ancient mechanics who captured the cosmos in bronze weren’t just modeling astronomical theories but were also inspiring them.