If I Am Right: Inge Lehmann and the Iron Heart of the Earth

At 10:17 on the morning of June 17, 1929, the ground split open in the hills behind Murchison, a small town on New Zealand’s South Island. The magnitude 7.3 earthquake tore up roads in the Buller Gorge, set off landslides that buried farms, and killed 17 people. It was a disaster for Murchison. For a woman working on the other side of the planet, it was also the start of one of the great discoveries of 20th-century science.

The shaking took well under half an hour to cross the Earth and reach the seismographs in Copenhagen. There, the records landed on the desk of Inge Lehmann, the head of seismology at Denmark’s geodetic institute. She was 41 and did her research in the evenings and on weekends because it was not part of her job. Reading the faint wiggles from Murchison and a few other big quakes, she found that something deep inside the planet was in the wrong place. In 1936 she published a paper, with one of the shortest titles in the history of geophysics, arguing that the Earth has a core within its core.

She was right. And nearly a century later, the iron ball she found is still surprising the people who study it.

Listening to a planet

Nobody has ever been anywhere near the Earth’s core. The deepest hole ever drilled, the Soviet Kola Superdeep Borehole, stopped about 12 kilometers down, and America’s Project Mohole never got through the crust at all. The core starts almost 2,900 kilometers below your feet. Everything we know about it comes from indirect evidence, and most of that comes from earthquakes.

The first hint was weight. In 1797 and 1798, Henry Cavendish used a torsion balance in an outbuilding at his house in Clapham, outside London, to measure how strongly lead balls attract each other, and from that worked out the density of the whole Earth: about 5.45 times that of water. The modern figure is 5.51. Surface rocks are only about half as dense, so something much heavier had to be hiding in the middle. Iron, which falls out of the sky in meteorites, was the obvious suspect.

Seismographs, which spread around the world in the 1890s, turned that guess into science. An earthquake sends out several kinds of waves. Compressional P waves push and pull the rock like a slinky and travel fastest. Shear S waves shake it side to side and arrive later. S waves cannot pass through a liquid, and both kinds bend and slow down when they cross into a different material. Clock the arrivals at enough stations and you can work out what the waves passed through.

In 1906 Richard Dixon Oldham, a British geologist who had spent years with the Geological Survey of India, used the arrival times to argue that the Earth has a distinct core that slows P waves down. In 1909 the Croatian seismologist Andrija Mohorovičić, studying a quake in the Kupa valley, spotted the boundary between crust and mantle that now carries his name, the Moho that Project Mohole set out to reach. In 1914 Beno Gutenberg in Germany pinned the top of the core at about 2,900 kilometers, remarkably close to today’s figure. And in 1926 the British geophysicist Harold Jeffreys showed that the core does not carry S waves. It had to be liquid.

By the late 1920s, then, the textbook picture was simple: a rocky crust, a solid mantle, and a liquid iron core all the way to the center. Inge Lehmann was about to complicate it.

A girl from Østerbro

Lehmann was born on May 13, 1888, in the Østerbro district of Copenhagen, the daughter of an experimental psychologist. She went to an unusual school run by Hanna Adler, an aunt of the physicist Niels Bohr, where boys and girls were taught the same subjects. “No difference between the intellect of boys and girls was recognized,” Lehmann wrote much later, “a fact that brought some disappointment later in life.”

She studied mathematics at the University of Copenhagen and spent a year at Newnham College, Cambridge, before overwork and a breakdown sent her home. She worked for years in an actuary’s office, crunching insurance numbers, and finally finished her degree in 1920. The number crunching turned out to be good training.

In 1925 she became an assistant to the geodesist Niels Erik Nørlund, who was setting up seismograph stations in Copenhagen and in Greenland, at Ivigtut and Scoresbysund. She helped install the instruments, and was put to work on them in a basement in Copenhagen with little instruction. “It wasn’t quite simple,” she remembered, “but when I had looked properly, I managed to figure out what to do. From that day forward, I worked with seismology.” In 1927 she spent a month learning from Gutenberg in Germany. The next year she was put in charge of the seismology department of the new Royal Danish Geodetic Institute, a post she held until 1953.

An earthquake in the wrong place

The core’s discovery had left a puzzle on the far side of the globe. Because P waves bend sharply downward when they hit the slower liquid core, there is a band of the Earth’s surface, roughly 105 to 140 degrees of arc from a quake, where direct P waves should not arrive at all. Seismologists call it the shadow zone.

The Murchison quake broke that rule. Stations in the Soviet Union, at Sverdlovsk, Baku and Irkutsk, sat inside the shadow zone and still recorded clear P waves. They were not strong, but they were there, and they were too sharp to wave away.

Other seismologists had noticed odd arrivals in the shadow and blamed them on waves diffracting, or bending, around the edge of the core. Lehmann did not think diffraction could explain them. She set out to collect every good record she could find.

The oatmeal boxes

This was work done entirely by hand. Each earthquake meant dozens of paper or photographic records from stations around the world, each one to be read, timed and corrected. Lehmann kept her data on cards and filed them in cardboard oatmeal boxes, one for each station: Tokyo, Vienna, London and so on. Nils Groes later recalled a summer day at her country cottage in Holte, north of Copenhagen, when “she sat on the lawn at a big table, filled with cardboard oatmeal boxes. In the boxes were cardboard cards with information on earthquakes.”

What she did with them was elegantly simple. The seismologist Bruce Bolt, who wrote her memoir for the Royal Society, called it an “existence theorem.” She built the plainest possible model of the planet: a mantle where P waves travel at 10 kilometers per second, a core where they travel at 8, and, inside the core, a smaller sphere where they speed up again. Then, using nothing more than straight-line paths and trigonometry, she showed that a fast inner sphere would bend some waves back out into the shadow zone, just where the Murchison waves turned up.

She called the core waves P′, pronounced “P prime,” and that was the title of her paper, published in 1936 by the International Seismological Bureau. It is a modest document. “A hypothesis will be here suggested,” she wrote, “which seems to hold some probability, although it cannot be proved from the data at hand.” She put the radius of the inner core at about 1,400 kilometers. The modern figure is about 1,220, roughly 70 percent of the Moon’s radius.

Privately, she was less modest. “If I’m right, and I know I am,” she wrote in one letter, “it seems that they have overlooked the most important part.” That line gave the title to Hanne Strager’s 2025 biography of her.

From hypothesis to fact

The idea caught on faster than many new ideas do. Gutenberg and Charles Richter, of the Richter scale, checked it against their own data in 1938 and agreed, though they put the boundary a little deeper. Jeffreys, who had argued for a liquid core, accepted the inner core in 1939 and showed that diffraction could not explain the shadow-zone waves.

Lehmann had shown the inner core was different from the outer core, not that it was solid. That came in steps. In 1940 the Harvard physicist Francis Birch argued that at the pressures near the center, iron should freeze even at enormous temperatures. In the 1940s the New Zealand–born seismologist Keith Bullen made the case from the way the Earth’s density and stiffness change with depth. Around 1970, seismologists using new arrays of instruments finally caught clean echoes bouncing off the inner core’s surface, and measurements in the early 1970s showed it was rigid, which is to say solid. Shear waves passing through it, the last missing piece of evidence, were not convincingly reported until 2005.

The picture we have today would have pleased her. The liquid outer core begins about 2,890 kilometers down. The solid inner core starts about 5,150 kilometers down and reaches to the center, at about 6,370. It is mostly iron and nickel, under 3.3 to 3.6 million times the pressure of the air at sea level, and is thought to be roughly as hot as the surface of the Sun. It is solid not because it is cool but because it is squeezed. And it is slowly growing, as the planet cools and the outer core freezes onto it, by something like a millimeter a year.

No time for gardening

Lehmann had to fight to be taken seriously for much of her career. She once complained, in a remark that has been quoted ever since, that she had had to compete with “incompetent men” for her work to be noticed. But by the 1950s the world had come to her. The Columbia University geophysicist Maurice Ewing invited her to the Lamont Geological Observatory in New York, and she spent long stretches there over the following years.

The Cold War helped. Governments that wanted to detect each other’s nuclear tests, the kind of test that went badly wrong at Castle Bravo, poured money into seismograph networks. In the late 1950s Lehmann, now officially retired, used the new data to find another boundary, about 220 kilometers down in the upper mantle, where waves suddenly speed up. It is known as the Lehmann discontinuity. “Retiring was a shock, but I recovered quickly,” she said. “The important thing is not to waste time or be engrossed in gardening.”

The honors piled up: the Bowie Medal of the American Geophysical Union in 1971, membership of the Royal Society of London in 1969, the medal of the Seismological Society of America in 1977. The Bowie citation called her “the master of a black art for which no amount of computerizing is likely to be a complete substitute.” She published her last paper in 1987, at 99. She died in Copenhagen on Feb. 21, 1993, at 104. “I’ve been thinking about my life,” she said near the end of her life. “It has been long, rich and fulfilling with many victories and many good memories.”

Like Cecilia Payne-Gaposchkin, who worked out what stars are made of in the same decade, she found the answer to a basic question about nature with numbers other people had already looked at, and then had to spend years waiting for the rest of her field to catch up.

The core that won’t sit still

Lehmann’s inner core turned out to be a much stranger object than a static ball of iron. It is the heart of the planet’s magnetic field. As it grows, the iron that freezes onto it leaves lighter elements behind in the liquid outer core, which rise and stir the outer core like a pot on a stove. That churning liquid metal is the dynamo that makes the field that shields us from the solar wind. In 1995 computer models by Gary Glatzmaier and Paul Roberts suggested that the magnetic forces should also nudge the inner core, making it spin at a slightly different rate from the rest of the planet.

The following year, Xiaodong Song and Paul Richards at Lamont, the same observatory where Lehmann had worked, reported in Nature that the inner core seemed to be turning slightly faster than the mantle. The idea has been argued over ever since. In January 2023 Yi Yang and Song, at Peking University, reported in Nature Geoscience that this extra spin had paused around 2009 and might be swinging back, part of an oscillation lasting about seven decades.

In June 2024 John Vidale and Wei Wang at the University of Southern California published the strongest evidence yet. They studied 121 repeating earthquakes near the South Sandwich Islands between 1991 and 2023, quakes that strike the same spot again and again and so send waves along the same path through the core. The waves showed the inner core began slowing down around 2010 and is now turning more slowly than the mantle. “When I first saw the seismograms that hinted at this change, I was stumped,” Vidale said. The effect on the length of our day is on the order of a thousandth of a second, far too small to notice, though it joins the family of tiny shifts in Earth’s rotation, like the Chandler wobble, that geophysicists track.

Then, in February 2025, the same group reported in Nature Geoscience that the waves suggested the surface of the inner core itself had changed shape in places, perhaps by 100 meters or more, possibly where the churning outer core drags on it. Vidale was careful to hedge: “We’re not 100% sure we’re interpreting these changes correctly.” Lehmann, who called her own great discovery a “hypothesis,” would have recognized the tone.

On the money

Denmark has not forgotten her. In September 2024 Danmarks Nationalbank announced that Inge Lehmann, along with the Earth’s core she discovered, will appear on one of a new series of banknotes entering circulation from 2028, alongside Hans Christian Andersen, the astronomer Tycho Brahe and the Greenlandic explorer Arnarulunnguaq. Lehmann had helped set up Denmark’s seismograph stations in Greenland, a place that would later host stranger science, like the Army’s Camp Century under the ice.

It is a fitting tribute for a scientist whose instrument was the whole planet. Every big earthquake still sends waves through the inner core she found, and seismographs around the world still catch them on the far side, in the shadow zone where, by the old textbook, nothing should arrive. Lehmann looked at that nothing and saw an iron heart.

Further reading