In 1909 an earthquake shook a valley south of Zagreb, and at stations far enough away the waves arrived twice. The man who worked out why was a meteorologist, and what he had found was the floor of the world.

Episode 5 of The Slow Machine is about how we mapped the inside of a planet nobody has ever been more than a few kilometres into. A network of seismographs built to record disasters, and the Earth rung like a bell. Two kinds of shaking, one of which cannot cross a liquid. A thin crust, a molten core the size of Mars, and a mantle that turned out to be solid, which seemed to finish Wegener off. Then a South African geologist with a bicycle and a camera who stood on both sides of the Atlantic, and an English physicist who dated the rocks, asked what the Earth's heat was doing, and wrote down the engine.

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 6, "Draining the Ocean" — a mountain range nobody knew was there, a sea floor with no old rocks in it at all, and a woman at a drafting table in New York who was not allowed on the ships.

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Guest voice: Jon W (Arthur Holmes), 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.