The surprise discovery came at the conclusion of a project that was spread over three years and a hectic two weeks — necessitated by limited financing and availability.
Among the first to spot it was Joy Appleton, who leads the Boxford History Project and who was a driving force behind the excavation.
“I was stunned into silence,” Ms. Appleton recalled of her first sight of the small red tiles, each the size of her fingernail. “Which is unusual.”
The expert on site, Matt Nichol, was equally surprised. “I will never forget that moment,” said Mr. Nichol, a professional archaeologist who was supervising the dig.
“It was down to the volunteers, it really was. I get quite emotional about it; it was something to see their drive,” added Mr. Nichol, project officer for Cotswold Archaeology, a company whose normal work includes helping real estate developers preserve archaeological finds.
Experts say the mosaic at what is now called Boxford villa depicts Bellerophon, a hero of Greek mythology who was sent to kill the chimera, a fire-breathing monster with the head of a lion, the torso of a goat and the tail of a serpent. Hercules is also thought to be featured, fighting a centaur, and so is Cupid.
According to Anthony Beeson, a specialist in classical art and member of the board of the Association for Roman Archaeology, the discovery is important for several reasons.
“It is so unusual because it has all sorts of quirks which you don’t expect, and it has subjects on it that are completely alien to mosaics in this country,” he said.
Some figures breach geometric borders and there seems to be a trompe l’oeil effect. Mr. Beeson added that he could not think “of another Roman mosaic in this country that is as creative as this one.” There are inscriptions, too, though only about one-third of the mosaic was excavated and the full text was not uncovered.
The execution is uneven, Mr. Beeson said, suggesting that the “mosaicist has had ideas above his technical ability,” producing what he called a “very sophisticated design done in a slightly naïve manner.”
Boxford villa had been marked — inaccurately, as it turned out — on an old map. (It later turned out that the site was disturbed in the 19th century, when the installation of a land drainage pipe damaged part of the mosaic.)