On 18 November 1929 an earthquake off Newfoundland snapped twelve transatlantic telegraph cables, one after another, over thirteen hours. Nobody could say what had broken them, because nobody knew what the bottom of the ocean looked like.

Episode 6 of The Slow Machine is about how we drew the sea floor. An echo sounder built to find icebergs and submarines, a Princeton geologist who never switched his off, and a Cold War surplus of instruments and money that sent ships out to measure everything. Mud that was far too thin, rocks that were far too young, and heat in the wrong place. Then a woman at a drafting table in New York, not allowed on the ships, who found a crack running down the middle of the Atlantic, and a telephone company's map of earthquakes that said she was right.

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 7, "The Tape Recorder" — a Nobel Prize-winner who built a machine to prove his own grand theory and instead proved himself wrong, two continents with different North Poles, and the most spectacular rejection letter in the history of the Earth sciences.

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Guest voice: Jane Goddard (Marie Tharp). 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.