Jacques Kornprobst, the man who can read the secrets of the rocks, was agitated. He had arrived twenty minutes early to pick me up at the hotel in Clermont‑Ferrand, an ancient French university town perched on an annealed crack in the planet’s crust. He had the entry code at the ready to get into the free parking lot behind the building. The code had failed him.

Some drivers cruise the streets nonchalantly, certain that the perfect parking spot will open up at just the right time. Kornprobst was not among them. Parking in this city of 150,000 had become trouble‑ some over the decades he had lived there, and as he had mapped out the day’s tightly choreographed itinerary he had made intricate plans about where to park. And now, the first parking spot of the day had fallen through.

Inside the hotel he sprinted, red‑faced, fingertips frigid in the spring chill.

“Kornprobst!” he rapped out as he met me for the first time. Then he turned swiftly to the reception desk to let off a stream of injured French, explaining to the bewildered woman sitting there—she had been so friendly earlier, solicitous about replenishing the croissant basket and tinkering with the café‑au‑lait machine—about the affront. He had called the day before to secure the code. And now, today, he said, chin thrust slightly forward, it was malfunctioning.

Abruptly, she left through a back door. He darted out front to a tiny blue Renault car that was parked haphazardly on a curve at the corner, performed a roundabout U‑turn through the city’s tortured roads, and then nosed up to the gate with its uncooperative code. The receptionist stood there, punching in numbers, shivering. He drummed his fingers on the steering wheel. Finally, the barrier be‑ gan to rise and the receptionist, without so much as a glance behind her, returned inside to her desk. Kornprobst smiled grimly, thrust the little car into gear, gunned the engine, and zoomed triumphantly into a parking spot.

He was watching the clock. He was on a mission to memorialize the life and work of Bernard Brunhes, a French physicist who, along with his research assistant Pierre David, made an astounding, violently unsettling, and controversial find at the turn of the last century. Brunhes, whose name is pronounced “brune,” discovered that the planet’s two magnetic poles—north and south—had once switched places. In the decades following his discovery, his colleagues, originally aghast at Brunhes’s finding, proved that the poles have reversed not just once, but many times on an unpredictable, or “aperiodic,” schedule. The last time was 780,000 years ago.

But despite the fact that our current magnetic epoch is named after him, Brunhes has largely slipped out of the scientific memory. He does not even rate his own entry in the Encyclopedia of Geomagnetism and Paleomagnetism, the bible of the discipline of reading patterns in the Earth’s magnetic fields. Nor is he lionized in France, usually so careful to honor its own. In fact, he’s all but unknown even in his homeland, along with his grand scientific finding that the poles can switch places, that up can become down.

Kornprobst, a fellow physicist, felt that he must right this wrong. He was so committed to Brunhes’s memory that some years ago he took the trouble to find the spot in the countryside where Brunhes hacked a piece of crumbly terracotta rock—similar to the stuff of Greek vases—out of a roadcut and made his great discovery. Kornprobst painstakingly pieced together the clues about where it could be and is one of a handful of people in the world who can usually find it. The first time he made the pilgrimage to the site, he left frustrated, having failed to identify the right seam of rock. He’s found it several times since, but it’s so overgrown, so unmarked, that success is al‑ ways touch and go.

Kornprobst thought that Brunhes should at least have a commem‑ orative panel at the university in Clermont‑Ferrand, so he sweated through a couple of years writing to geological agencies and eminent physicists all over the world jostling their elbows about Brunhes’s contribution to science, raising the money to erect it. Then he arranged for a ceremony and lecture to accompany its inauguration at the university in 2014. It was through that ceremony that I found Kornprobst. He wrote an article about it for Eos, a journal of the American Geophysical Union. I read it and sent him an email asking him if he would help me understand why Brunhes was so important and maybe even find that seam of terracotta. He wrote back thirteen minutes later to say he would be delighted. I was at the hotel in Clermont‑Ferrand two weeks later.

Sporting a thick, off‑white cable‑knit sweater the same hue as his rakish hair, Kornprobst left the car in the lot and we set off briskly on foot from the hotel through the back streets of Clermont‑Ferrand. It is one of the oldest cities in France, founded more than two millennia ago on the site of what was then a sacred grove of trees. And so we were marching through time, across the history of science. Up the road named after Pierre Teilhard de Chardin, a Jesuit priest and paleontologist who deeply offended the Vatican for asserting that the book of Genesis is more allegory than fact. Past the geology depart‑ ment of the downtown campus of Université Blaise Pascal, named after the seventeenth‑century mathematician and physicist whose seminal experiment on barometric pressure was conducted a few kilometers outside the city by a brother‑in‑law (“There is the belief that Pascal experimented with pressure here,” Kornprobst declaimed, pointing vigorously down the street, “but it’s not true!”). Across a road named after the nineteenth‑century zoologist Karl Kessler. And finally, to rue de Rabanesse, named after the tiny pale stone Renaissance castle that was Brunhes’s home and first observatory.

Kornprobst gestured to it triumphantly, eyebrows raised, as if it explained a great deal.

It looked like nothing out of the ordinary. It was standing forgotten on an overgrown patch of land across the street from a busy art school, surrounded by two layers of forbidding wire fence. Many of its lower windows—once elegant—were partially filled in with cement blocks. The parging that had covered the volcanic fieldstone that made up its walls had decayed, leaving gaps along the seams so you could see how it had all been fitted together. Its turret, where Brunhes collected meteorological information beginning in 1900, was still sturdy, reaching six floors into the sky, fifteenth‑century iron fretwork still robust.

This observatory is where the tale of Brunhes begins. And where the tale of Brunhes begins, so too does the story of the discovery of the planet’s long string of pole reversals. And that story, in turn, contains the tale of the mysterious magnetic organism in the core of the planet and how it has become deeply disturbed once more, yet again deciding whether to reverse.

It was here that Brunhes, whose name means “brown” in the Occitan language of the ancient troubadours of this land, began to dream of understanding magnetism, the Earth’s secret power. We never feel it and rarely see it, but all the same, scientists and philosophers have been trying to understand it for thousands of years. For most of that time, people have imagined it to be local and transient. Magic, even. And fickle magic at that. In fact, magnetism is one of the few essential powers of the universe. To understand it, you have to go back in time to the birth of the universe, to see how the universe is arranged. And you have to do that in the company of theoretical physicists, who have developed the most precise mathematical laws so far to describe reality.