THE ODD SALON PODCAST

Inge Lehmann sat alone in a refurbished concrete bunker on the western flank of Copenhagen Denmark, staring at her fantastic machines. The smell of wet wool and damp earth trod in on the souls of her boots permeated the still air, and seemed to pool atop the arched ceiling. She was waiting for a catastrophe on the other side of the world.

Lehmann was a geologist. She studied earthquakes and the study of geology and earthquakes depended on human tragedy. The first indication that the catastrophe had occurred would be an oscillation, a steel rod suspended between springs would sway back and forth. Without this catastrophe, our study of planets and their interiors would continue to be wrong, confounding scientists trying to determine how planets form, how continents move, and how life originated on our planet. 

If the catastrophe did occur, Lehmann knew, people would lose their lives. She watched her machines. She was still as a vision with her high bony cheeks under a messy Victorian updo. Behind her deep set, gray eyes, Lehmann, now in her early forties reviewed her research of the past three years. Was she right?

Did she have the answer that had eluded her fellow scientists? As time drew out, like one of the tightly wound springs, on June 17th, 1929 the steel rod began to move.

I’m Miles Traer. I’m Odd Salon’s resident geologist. And on this episode, I’ll be talking about how a woman, an earthquake, and a cardboard oatmeal box changed the world.

Three years earlier, Lehmann stood within the same concrete bunker for the first time. She’d recently be given the title of Chief of the Seismological Department at the University of Copenhagen, a lofty sounding epithet that was little more than ceremonial. The university didn’t care much for what she did out here at the edge of the city, her nephew remarked “It was not easy for a woman to make her way into the mathematical and scientific establishment. In the first half of the 20th century.” 

It was a dramatic understatement. Added to this was the general belief that the study of earth its geology and earthquakes was the stuff of lesser scientists: true scientists like Isaac Newton before turned their gaze to the heavens, not to the rock beneath their feet.

These combined sentiments taunted Lehmann.

For years, she had struggled to gain respect as a scientist, struggled to find a position that would allow her to practice scientific research – and geology, specifically the study of earthquakes, had provided this haven. Lehmann’s boss had told her that earthquake waves could help reveal Earth’s inner structure, like the newly discovered x-ray, if only they could be properly read. The hook had been set. The answer to impossibly grand questions lay within reach: what was down there, how did the earth form, and who would be first define the answer? Only a handful of people across the world took up this call; all of the men. Step one for Lehmann was to contact this handful of men and see if they would meet with her.

As she waited for the responses, she dove into her research. She began with what happens during earthquakes, how the ground shaking sends waves riping through the planet like a bell and how the waves can bend in long arcs or even bounce off of surfaces deep within the planet. The physics made sense to her. The math created what scientists call a model, a series of equations like E equals MC squared that helped explain what Earth’s interior looked like. The mathematical models showed that the waves bent and moved, and because of this, some waves would show up in certain locations and others would not.

What perplexed her was why some of these waves showed up where they shouldn’t, where geologists call the “Shadow Zone”. Lehmann was a virtuoso scientific observer, and this didn’t sit right with her. 

Something was wrong with mathematical models of her male colleagues. Why were these waves showing up in the Shadow Zone? She made a note to ask one of the men on her list in the summer of 1927, the first letters came back to Inge Lehmann. The men agreed to meet with her. Her travels would take her all over Europe, but the most important stop was in Darmstadt, Germany where she met Beno Gutenberg. Gutenberg, age 38, was already legend in the study of earthquakes.

The field’s equivalent of Galileo, Gutenberg’s observations had helped define the new science and he had created the equations by which his fellow scientists measured the precise timing of the earthquake waves. After just their first meeting layman and Guttenberg established their working relationship.

Their conversation was direct, critical, stimulating, and challenging. They became instant friends, perhaps it was his own experiences with antisemitism that allowed him to empathize with this extraordinary woman. Similarly, outcast and isolated from her fellow scientists.

While the other men on her list had given her a few days of their time, Gutenberg offered much more. “I spent some months with him there,” Lehmann remembered. “He gave me plenty of his time in a most unselfish manner. I could have had no better introduction into seismology.” In Gutenberg, layman found her missing mentor.

He began to teach her the language of earthquakes inscribed in the squiggly lines on the rolls of smoked paper. “But what of the waves showing up where they shouldn’t?” Lehmann pressed. 

“It was nothing,” Gutenberg assured, “people miscalculating the time.”

“But they’re using your equations,” Lehmann continued. 

“Eh, it’s probably some mistake in the way the researchers recorded the arrival of the waves,” Gutenberg offered. 

Something still didn’t make sense. A year later, Lehmann met one of Gutenberg’s greatest scientific rivals, an Englishman named Harold Jeffreys. Jeffreys, like Gutenberg, calculated how earthquake waves traveled through the planet and created his own mathematical model. Again, Lehmann raised the question: why were these waves showing up where they shouldn’t? Like Gutenberg before him, Jeffreys dismissed it. Other scientists must have been doing something wrong, he insisted.

Lehmann went back to Copenhagen, determined to make sense of this puzzle. The waves kept showing up. An idea began to form in Lehmann’s head. For her to investigate this idea she needed an earthquake as close to the other side of the world as possible; this ensured that the waves would have to travel directly through Earth’s center and that her Copenhagen station would be firmly within the Shadow Zone.

She did the calculations: it needed to be big. For the waves to make it that far and still carry the signal she hoped to see, the earthquake would have to rival that of the infamous 1906 San Francisco earthquake, which destroyed the city and killed some 3000 people. 

She looked to her globe and her finger followed the gentle curve to the point opposite Denmark: New Zealand. The quake she needed would have to happen near New Zealand. So she waited, waited for a catastrophe on the other side of the world. 

On June 17th, 1929, a series of heavy booms echoed across the valley surrounding the rural town of Murchison.

At 10:17 in the morning, the ground lept. One survivor recalled, “There was a terrific roar. And we wondered if it was the end of the world.” The rocky ridges surrounding Murchison cracked and a landslide, the size of a mountain roared through the town obliterating a farmstead along the way. Another landslide rushed through a school where, miraculously, all the children escaped by swimming through the mud and the debris before finding shelter in a nearby tree. A boulder the size of a barn broke away from its cliff and flattened a house near the river, killing the inhabitants immediately.

And the river itself became blocked by the debris, building up a lake that threatened to burst through the makeshift dam and swallow the town. Then the rains came. 

11,000 miles away in a concrete bunker in Copenhagen, Denmark Inge Lehmann watched a steel rod suspended between two springs begin to move. The oscillation. She took a red pencil and marked the spool of paper next to the squiggly lines to indicate an earthquake. “New Zealand” it read.

Lehmann immediately dove into the calculations. She used Gutenberg’s equations. She used Jeffreys’. The waves that weren’t supposed to be there in the Shadow Zone were definitely there. This wasn’t a mistake. This wasn’t a miscalculation. The mathematical models were wrong. 

Not long after she began her work she had the evidence to support her theory. She took out a piece of paper and drew a line from the earthquake in New Zealand toward the center of the Earth – but before the line reached the center, she drew a sharp bend, like a rock skipping off the surface of a pond, the pencil traced back to the surface and intersected at Copenhagen. There was something down there that nobody had seen before. Something missing from Gutenberg and Jeffreys’ equations. She kept drawing. She drew a series of nested circles representing her discovery: a crust, a mantle, a liquid outer core, and a solid inner core. While this is familiar today and it’s taught to school children all over the world, this was the first time anyone had ever written that down. And it was Lehmann who had found the missing inner core. The waves were traveling from the earthquake and bouncing off of the core. This is why they were showing up where scientists weren’t expecting them to. She had the answer.

But now Lehmann faced a different problem. She had to convince the geology community that they were wrong. To do that she knew would take time. It ended up taking close to a decade. 

Her first report about the earthquake, published as little more than a series of facts, like depth angles, arrival times, didn’t mention the core. Her second report also omitted this crucial observation. She published a study about a different earthquake and still didn’t mention the core, but each study established critical information she knew she needed to convince her fellow geologists. She mentioned bent waves in reference to the waves in the Shadow Zone. She established the reliability of her machines and her observations. She established the arrival times of the waves. She slowly built her case. It wasn’t until 1932, three years after the earthquake, when she confided in Harold Jeffreys that she had found something.

In a letter to Jeffreys, she included a mention, right at the very end, basically a footnote. She didn’t mention the core explicitly. Instead, she mentioned the boundary between the core and the surrounding material, what geologists call a “discontinuity”.

Jeffreys wrote back, “Sorry about the new discontinuity – as if we hadn’t enough trouble already.” With the clear intonation that he didn’t believe it was real. Lehmann persisted. Near the end of the year, she wrote Jeffreys again. “I feel quite sure about the bend,” she wrote. Jeffreys again rejected the idea. She approached Gutenberg who also thought Inge must have been doing something wrong.

Even her closest friends didn’t believe her, but it didn’t matter. She persisted. It took Lehmann four more years. But she eventually published her work and changed the world. The title to her study like Lehmann herself was modest and unassuming: “P Prime”. That was it. “P Prime”. This was the naming convention that geologists used to label different earthquake waves.

And P Prime was the code that geologists used to label the waves that weren’t supposed to be there. On page two, she included her drawing of Earth’s layers, including her inner core. 

“There was a delay of some, two years before leading seismologists at the time could bring themselves to accepting the fact that a woman could make such a major step forward,” remembered her colleague years later. Not even her closest friends, Gutenberg and Jeffreys accepted her model right away. Guttenberg revisited Lehmann’s observations years later, trying to find fault in her methods.

He didn’t find any, but that didn’t stop him from suggesting that Lehmann’s core wasn’t really a solid core, more like a gradual shift from liquid to solid. Several years after that, though, he finally relented and adopted Lehmann’s core. Jeffries was slower to accept that a core existed, but he agreed that whatever Lehmann had found did have a sharp boundary in 1939, three years after Lehmann’s P Prime Jeffreys finally adjusted his own equations to account for Lehmann’s core. Today, using advanced techniques and equipment seismologists have determined the exact shape and size of the inner core.

Lehmann, working from limited information and recording her data on scraps of cardboard that she stored in oatmeal boxes, had been correct to within just 100 miles. Not bad, considering that it’s over 3000 miles down.

In the decades that followed scientists used Lehmann’s model to explore how the core’s radioactivity keeps the planet warm and how the core’s immense heat stirs the liquid metal and generates our planet’s protective magnetic field, allowing for life to thrive on its surface. This model helped scientists working in the 1950s and 60s explain how Earth’s surface was broken into large plates and how they moved, giving birth to the theory of plate tectonics. And in more recent years, scientists have adopted this model to explain how the planets of our solar system formed even measuring Mars’s core for the first time in 2021. Even today layman’s P prime remains a remarkable piece of scientific research; a reminder of what’s possible with careful observations and an adherence to science. Her legacy lives on in every one of us who remains curious, keeps asking better questions informed by scientific observations and wonders what’s possible when we stare into the spectacular unknown.