Amateur archaeologists in England have unearthed a stunning Roman dodecahedron — a mysterious class of objects that has baffled experts for centuries.

The dodecahedron — a 12-sided metal shell about the size of a grapefruit — was found this past summer during a dig in a farmer’s field near the Lincolnshire village of Norton Disney, located about 35 miles (56 kilometers) southeast of Sheffield.

A previous geophysical survey, which had detected underground areas where the Earth’s magnetic field had been disturbed, had revealed what looked like a buried pit at the site, and metal detectorists had already found Roman coins and broaches in the same field, said Richard Parker, the secretary of the Norton Disney History and Archaeology Group, an organization of local volunteers.

Parker was making a cup of tea nearby when a shout went up from some of the volunteers, who’d just unearthed the dodecahedron in one of the trenches the group made at the site for the two-week dig.