Robb’s The Discovery of Middle Earth: Mapping the Lost World of the Celts takes a look at how the indigenous, pre-Roman people of Europe plotted their whole world according to a complex web of “solstice lines.”

Early on, you mention the spectre of “historical hallucinations.” What was it like to uncover this phenomenon?

The discovery took me completely by surprise. In 2008, I was starting work on a very different book, and the idea that the Celts had mapped their world with mysterious pathways was an unwanted distraction; a pseudo-scholarly fantasy. I spent months trying to prove that it couldn’t have been true. But so many other verifiable discoveries came to light that, after a first exploratory year in the library and on the roads of Celtic Europe, I seemed to have entered an unknown world with a curiously accurate map in my pocket.

Can you touch upon the relationship between myth and history in the book?

The Celts kept meticulous records, spanning centuries, of astronomical events, wars, and migrations. They were fascinated by their own history and preserved it in legends which often match the archaeological record quite closely. The key is to confront different kinds of evidence instead of considering myths and legends in isolation. Some of those misty tales turn out to be as precise as the equations that produced the labyrinthine designs of Celtic art.

How was Rome able to smother so much of the Celtic culture?

The smothering of Celtic culture was partly a result of deliberate genocide and enslavement. But this is also an illusion created by the fact that the history of Roman domination was written by Roman historians. The image of the Celt as a drunken, self-destructive, hairy Highlander is a creation of Roman propaganda. When the legions marched over the Alps and crossed the English Channel, they found a sophisticated, literate society with democratic institutions, high-speed roads, and some of the most advanced technology in the ancient world. Unlike the Romans, the Celts rarely built in stone, so the remnants of their world tend to be no more noticeable than a gold coin in a ploughed field. But Celtic culture survived to flourish once again in the early-Christian church.

There are a lot of fascinating figures from this world. Do you have a favorite?

I was captivated by Diviciacus the Druid. The Druids are supposed to have been primitive, white-robed priests who lived in the gloom of oak woods, performing human sacrifices. Diviciacus was a diplomat and a politician who stayed at Cicero’s house in Rome. He was a family man, a student of astronomy and natural science, and a personal friend of Caesar.

Where do scholars and amateur enthusiasts go from here?

I could have spent a decade or more following the paths this opened up, but I wanted to publish what I had discovered. So much else remains; the system has major implications for the history of religion, science, urbanization, map-making, and exploration. This is just the first word on the subject.