Today in History—March 29: A Woman Discovers Earth’s Core

The Danish seismologist who discovered Earth’s inner core built her theory using cardboard oatmeal boxes filled with index cards—each recording a seismic event—and she did the math by hand in her sunny garden.

In doing so, Inge Lehmann changed how geophysicists understood the structure of the Earth.

Lehmann began her studies at the University of Copenhagen and later, in 1910, enrolled at the University of Cambridge—where women were barred from most libraries and lectures. Even if she passed her exams, she wouldn’t be granted a degree from Cambridge due to her gender. So she went home and finished her studies in Copenhagen.

After graduating, she joined the Royal Danish Geodetic Institute. As the only seismologist in Denmark for 25 years (her joke), she built the field almost from scratch, establishing seismograph research stations across the country and in Greenland.

Colleagues around the world sent her meticulous seismic records. She sorted them, compared them, and logged them.

Then came a 1929 earthquake in New Zealand.

Earthquakes produce different kinds of waves. Lehmann focused on P (primary, or pressure) and S (secondary, or shear) waves. P waves pass through all states of matter; S waves travel only through solids.

At the time, Earth’s core was believed to be a single liquid mass. But during the 1929 New Zealand quake, two stations recorded unusually strong P waves, which, Lehmann noted, shouldn’t happen in a fully homogenous core. Some P waves had been deflected, as if something inside the planet had bent them. The only explanation, Lehmann concluded, was a solid inner core.

On this day in 1936, she described the idea in a letter to Harold Jeffreys, a statistician, a leading authority in geophysics, and a friend. He wasn’t convinced.

She gathered more data, more quakes, more anomalies—until the pattern held. That September she published a paper about it, modestly titled P’. Her insight established that Earth’s core has distinct inner and outer layers. The scientific community soon came around to the theory, including Jeffreys. (He and Lehmann remained lifelong friends and correspondents.)

And eventually, technology caught up—showing just how clearly Lehmann had mapped a place no one had ever seen.