In 1950 a Nobel Prize-winning physicist built the most sensitive magnetic instrument in the world, to prove that anything massive becomes a magnet simply by spinning. There was no field. He was wrong. What he had left was the instrument, and it turned out to be the most useful object in twentieth-century geology.

Episode 7 of The Slow Machine is about the rocks that remember where north was. Lava that freezes millions of microscopic compass needles in place as it cools. Two continents whose rocks disagree about where the North Pole used to be. A magnetic field that turns itself upside down, again and again. Then a submarine detector towed behind a survey ship, a map of the Pacific floor striped like a barcode that nobody could explain, and two ideas from two different fields put side by side. Along the way, the most spectacular rejection letter in the history of the Earth sciences, and a prediction made in public that could have failed.

The Slow Machine is a nine-part series on how we found out that the continents move, and why almost every clue came from an instrument built for something else entirely.

Next time: Episode 8, "The Bomb Detectives" — how a global network of listening posts, buried in sixty countries so that the superpowers could sign a treaty about nuclear weapons, found the place where the world goes to die.

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Guest voices: Tom H (Harry Hess, and the referee), found on Fiverr. Every quotation is listed with its source on this episode's page at theslowmachine.com.

Written and narrated by Michael Goddard. Music composed with Suno. Sound effects from freesound.org, all Creative Commons 0. Every voice in this series is human.