All Luke Irwin wanted to do was install a light in his barn, so his kids could play ping pong in the evening. But when workers dug around his Wiltshire farmhouse to lay electrical wires they unearthed an archaeological bonanza.

Irwin describes to As it Happens host Carol Off how one afternoon, a workman was digging a trench about a foot-and-a-half deep, when he cried out "I've hit something! I don't know what it is! There's lots of colour here, and it's hard and it's flat."

"We brushed back the mud, and you could see this brilliant, vivid colour," says Irwin. "Slate-blue, chalk-white, and terra cotta. It had the vividness of the day it was laid."

It was in fact a mosaic, at least 1,500 years old, now believed to have adorned the floor of a very "high-status" Roman villa.

"When they use the word 'villa'," explains Irwin, "that is actually a catch-all phrase for a Roman country house. And these can be modest, these can be large — this one is I think what you might call 'palatial.' The scale of the building, from what I understand, is about 100 metres by 100 metres. Certainly three stories tall. They had under-floor heating. They had running water on the second and third floor. This was the highest status form of house in Britain at the time."

Irwin says it likely belonged to a British aristocrat family living under Roman rule. And he points out that, like many buildings of the time, the villa was likely built in stages — with extensions added as the family's wealth increased. "They think that the initial building was begun in about 175 A.D. And this mosaic and this palatial scale probably happened between 300 or 350 A.D."

Archaeologists are already hailing the find a "national treasure" — mostly, says Irwin, because of how immaculately preserved it is.

"In the last thousand years, this site has only been owned by three different entities. Firstly, it was Queen Maud, who was William the Conqueror's wife, and she gave it to a convent in Normandy. The convent gave it to King's College Cambridge in 1460. And they owned it from then until 1960. So you've had absentee landlords, you've had minimal farming, which is the greatest destroyer of archaeology."

What's more, the archaeological dig has revealed evidence of pre-farming habitation from the Mesolithic era — 10,000 B.C.E.

Other dramatic finds include a child's coffin from the Roman era, and oyster shells and animal bones that further suggest the inhabitants were aristocrats who enjoyed a high-status diet.

"On a cold, drizzly February afternoon, it's hard to imagine someone in a Toga having a particularly happy time," says Irwin. "But when you start thinking that somebody has lived where you live for 12,000 years, it is bewildering. It is utterly belittling and it is utterly magnificent."