There Was Another Ball at the Center of the Earth

The listener is a child; the reader is a parent. This page is here so you can answer the questions your child asks after the story. The script is available during playback via the script button.

What this is about

No one has ever been to the center of the Earth. Yet science books today draw the Earth with four layers: crust, mantle, core and inner core. The last of those lines, a second core inside the core, was first put forward by the Danish seismologist Inge Lehmann (1888–1993) (DOI: 10.1029/eo068i046p01593-04).

This piece follows the person, not only the finding. She wanted to study mathematics, but doors were closed to women, so she changed her path. For 25 years she was the only seismologist in Denmark, and she read the records alone. And in 1936, in a paper titled just "P′", she wrote down that "it cannot be proved from the data at hand."

The four parts of "People who measured the Earth" (Eratosthenes, Lehmann, Wegener, and the Alvarez father and son) share one question: how do you measure a place you cannot visit, from the traces it leaves? For Lehmann, the traces were the records of shaking left by earthquakes.

What actually happened

• She was born in Copenhagen on 13 May 1888. Her childhood was happy. Her father, Alfred Lehmann, was a professor of psychology at the University of Copenhagen who started experimental psychology in Denmark. He was rarely seen except at meals, though sometimes on Sunday he took the family for a walk. She lived on Kastelvej in Copenhagen for over 50 years (DOI: 10.1098/rsbm.1997.0016).
• At fifteen or sixteen, she felt an earthquake at home on a Sunday morning. This is her own memory. The floor moved and the hanging lamp swayed, and her father said, "It was an earthquake." The movement felt slow, so the center was probably far away. She wrote that this was her only experience of an earthquake until she became a seismologist 20 years later (DOI: 10.1029/eo068i003p00033-02).
• Doors closed to women. She had gone to a forward-looking school for boys and girls, so she had little direct experience of unfair treatment. After studying mathematics in Copenhagen, she went to Cambridge in 1911 and was not ready for the way men and women were kept apart there. Worn out from overwork, she gave up her studies and went home (DOI: 10.5194/hgss-2021-20). Her letters to Niels Bohr record her disappointment with the limits on women at Cambridge. A researcher thinks her breakdown in the winter of 1912 likely came from working too hard to overcome bias against women (DOI: 10.5194/hgss-13-83-2022).
• A degree, and then an office job. After a Danish degree in mathematics, she became an underpaid office worker at the university (DOI: 10.5194/hgss-13-83-2022). In 1923 she became an assistant to a professor of insurance mathematics (DOI: 10.5194/hgss-2021-20).
• The choice to change path. Her chances of a science post at the university were slim, so she joined Professor Nørlund, who was reforming the Danish survey service (DOI: 10.5194/hgss-2021-20). In the researcher's words, only by changing her field from respected mathematics to little-known seismology could she become a successful scientist (DOI: 10.5194/hgss-13-83-2022).
• A station in a fort. In the autumn of 1925 she became Nørlund's assistant. Her first big task was to help set up seismographs in two solid buildings of the old fort around Copenhagen, away from shaking such as traffic. The station opened on 18 November 1926 (DOI: 10.1029/eo068i003p00033-02).
• A network that reached Greenland. She set up a wide network with stations in Copenhagen and at Ivigtut and Scoresby Sound in Greenland. In 1928 she became head of the seismology department of the Royal Danish Geodetic Institute, and she held that post until she retired in 1952. In those years she read all the records herself and published the station reports (DOI: 10.1029/eo052i007p00537). For 25 years she was the only seismologist in Denmark (DOI: 10.1029/93eo00411).
• What was known before her. In 1906 Oldham found the line between core and mantle. Gutenberg set its depth at 2900 km. In 1926 Jeffreys answered the case for a solid core and showed that the core is liquid. The same source notes that the missing S waves alone did not convince seismologists of that time that the core was liquid, "contrary to modern textbook statements" (DOI: 10.1119/1.12026). So the story gives the shadow as "how science books explain it today" and tells the history apart. So just before Lehmann, the picture was "one liquid ball in the middle."
• On 16 June 1929 (world time), the magnitude 7.8 Murchison earthquake struck the South Island of New Zealand (DOI: 10.5459/bnzsee.27.3.190-204). In local time it was the morning of the 17th (DOI: 10.1017/s0016756800099027). In 1930 Lehmann published "P' as read from the records of the earthquake of June 16th 1929" (openalex:W572547897 — title only, no abstract). Taking this paper's earthquake to be the Murchison earthquake is an inference from the matching dates (see "What is evidence" below).
• 1936: "P′". Lehmann published a paper with the one-word title "P′" in the science reports of the international seismology bureau (BCSI). That title made her a favorite of people who compile lists of papers (DOI: 10.1029/eo052i007p00537). The paper says: "An explanation of the P3′ wave is required, since now it can hardly be considered probable that it is due to diffraction. … We take it that, as before, the Earth consists of a core and a mantle, but that inside the core there is an inner core in which the velocity is larger than in the outer one." (DOI: 10.1029/eo068i006p00073-01; DOI: 10.18814/epiiugs/2001/v24i4/007)
• And she wrote down her own limits. In the same paper she wrote that the idea "seems to hold some probability, although it cannot be proved from the data at hand" (DOI: 10.1029/eo068i006p00073-01), and that "the assumption of the existence of an inner core is, at least, not contradicted by the observations; these are, perhaps, more easily explained on this assumption" (DOI: 10.1017/9781139583954.002).
• An idea that began before 1936. Her early letters with Jeffreys already talk about the inner core in 1932 (DOI: 10.4401/ag-4625).
• "Solid" came later. What Lehmann put forward was "an inner core where waves go faster." The claim that the inner core is solid was made later by Bullen, based on likely ideas about how hard it is to squeeze and how dense it is. Later, the study of the Earth's free oscillations gave fairly convincing evidence. Attempts to find S waves that pass through the inner core had failed when that history was written, with one possible exception (DOI: 10.1119/1.12026). Several papers (DOI: 10.1029/95eo00143; DOI: 10.4169/college.math.j.44.5.399) say in short that "Lehmann found a solid inner core." But the 1936 paper itself says only that waves go faster there.
• Before and after she retired. She helped create the European Seismological Commission in 1951 and the International Seismological Centre in 1964. After retiring she worked at home in Denmark, and abroad in the USA and Canada (DOI: 10.4401/ag-4625). One source gives her retirement year as 1952 (DOI: 10.1029/eo052i007p00537), another as 1953 (DOI: 10.4401/ag-4625). In the 1950s she came to work at Lamont in the USA. A friend she made there remembers her as very shy. She cooked her own meals in the guest house and kept the ice cream and frozen food her friend had bought in her own small freezer until the friend went home after work (DOI: 10.1029/93eo00520).
• Honors. In 1971 she received the William Bowie Medal of the American Geophysical Union (AGU) (DOI: 10.1029/93eo00411). The medal citation said her finding came from "exacting scrutiny of seismic records by a master of a black art for which no amount of computerizing is likely to be a complete substitute" (DOI: 10.1029/eo052i007p00537). A piece looking back at the 50th year of the finding wrote, "In atomic physics an equivalent act would warrant a Nobel prize" (DOI: 10.1029/eo068i006p00073-01).
• She died on 21 February 1993 (DOI: 10.1098/rsbm.1997.0016).

Questions your child might ask

"No one went inside the Earth. How do we know?" Because the shaking from an earthquake passes through the Earth and travels far. Collect when the shaking reached stations around the world, and where it arrived and where it did not, and you can work out the path it took (DOI: 10.4169/college.math.j.44.5.399). It is the same idea as shaking a closed box to guess what is inside.

"What is the shadow?" One kind of shaking (S waves) cannot pass through liquid. So after an earthquake, a wide area on the far side of the Earth, beyond about 100° from the quake, gets no S waves, and there is also a gap where P waves are weak. Textbooks today explain this as direct evidence that the outer core is liquid (in history, this alone did not convince people; see above). After the 2025 Kamchatka earthquake, high school students in Italy saw this shadow for themselves using data from a school seismic network (DOI: 10.5194/egusphere-egu26-2627).

"Is the inner ball hard?" That is how we see it today. But it was not Lehmann who said so. It is evidence that came later (DOI: 10.1119/1.12026). Several papers (DOI: 10.1029/95eo00143; DOI: 10.4169/college.math.j.44.5.399) say in short that "Lehmann found a solid inner core," but the 1936 paper itself says only that waves go faster there. You can tell your child: "Lehmann said shaking goes faster there. Other people added later that it is solid."

"Do we know everything about that ball now?" No. The days of seeing the inner core as a simple solid ball inside the liquid outer core are over. Today it is studied as something complex, with properties that change with direction and small features near its top (DOI: 10.1002/2014rg000469). At 98, Lehmann herself wrote, "the last word has probably not yet been said" (DOI: 10.1017/9781139583954.002).

"What does that ball have to do with us?" Scientists have long asked what part the inner core may play in making and keeping the Earth's magnetic field (DOI: 10.1029/95eo00143; DOI: 10.1002/2014rg000469). That is the field that moves a compass needle. Please tell your child that the question is still "may it play a part?", not "it does."

"How old was she when she died?" This is a good one to count together. She was born on 13 May 1888 and died on 21 February 1993 (DOI: 10.1098/rsbm.1997.0016), so she was 104. But two notices from that time said "105" (DOI: 10.1029/93eo00411; DOI: 10.1029/95eo00143). The story counts this out with the child on purpose.

Ideas in the story

• Measuring by traces: reading a place you cannot visit from records left by things that passed through it. This links all four parts of the set.
• Saying only as much as you know: the 1936 paper wrote both "it cannot be proved" and "it fits better."
• Changing path at a closed door: Lehmann did not dream of earthquakes from the start. It was a choice she made when the road was blocked.

What is evidence and what is a told story

What the sources state: her birth and death dates and family, her teenage earthquake (her own memory), the setbacks at Cambridge and the breakdown in the winter of 1912 (which the researcher calls "likely"), the low-paid office job and the change of field, the station in the fort and its opening in 1926, the Greenland network, reading all the records herself, 25 years as Denmark's only seismologist, the 1936 "P′" paper and its words, her work after retiring, daily life at Lamont, and the Bowie Medal and its citation.

What the story says we "could not check"

• ⚠️ "Lehmann found waves where no waves could arrive." This is the best-known version. The sources we have say only that "an explanation of the P3′ wave is required" and that it is unlikely to be diffraction. None of them says that the wave arrived inside the shadow zone. Textbooks often tell it that way, but the story says only what we could check and says plainly that we could not check the rest.
• ⚠️ That the 1929 New Zealand earthquake started the finding. The evidence goes as far as this: the date of the Murchison earthquake (DOI: 10.5459/bnzsee.27.3.190-204) matches the date in the title of Lehmann's 1930 paper (openalex:W572547897). None of the abstracts we have links the 1936 idea to that earthquake's records. The 1936 paper is described only as an "analysis of P-wave data" (DOI: 10.18814/epiiugs/2001/v24i4/007). The story says the same.
• ⚠️ Cards kept in oatmeal boxes. We looked from two angles and found no source for the story that Lehmann kept her earthquake notes on cards in oatmeal boxes. What does have a source is that thousands of her earthquake work papers came from her home and have been kept and scanned (DOI: 10.4401/ag-4625). The story tells that instead.

Kept for parents only

• The Cold War and nuclear test watching. Lehmann's private letters and records from the USA and Denmark show that American military agencies tried to bring Danish seismologists and the stations in Greenland into efforts to detect atomic bomb tests from far away. Denmark's policy on arms made that relationship more complex (DOI: 10.1057/978-1-137-59688-8_8). This does not fit the line of the child's story, so we left it out.
• Mixed-up names: the "Lehmann discontinuity." The 1971 medal citation and a 1987 proposal used "Lehmann discontinuity" for the edge of the inner core (DOI: 10.1029/eo052i007p00537; DOI: 10.1029/eo068i046p01593-04). But Lehmann also put forward a line in the upper mantle in 1964 (DOI: 10.7146/kvant.167669), and in seismology papers today "Lehmann discontinuity" usually means this upper mantle line, around 200 km deep (DOI: 10.1029/2004gl022333). If your child hears the name somewhere else, check which one is meant.

Sources and what came after

Key sources

• Inge Lehmann's paper: P'(1936) — words from the 1936 paper and why it mattered. DOI: 10.18814/epiiugs/2001/v24i4/007
• 50 years of studies on the inner core (1987) — words from the 1936 paper, "a Nobel prize." DOI: 10.1029/eo068i006p00073-01
• On the History of Inner Core Discovery (2017) — Lehmann's words from 1936 and 1987. DOI: 10.1017/9781139583954.002
• Discovery of the Earth's core (1980) — history of the core, and the order of the case for a solid inner core. DOI: 10.1119/1.12026
• Seismology in the days of old (1987) — Lehmann's own memories. DOI: 10.1029/eo068i003p00033-02
• Inge Lehmann. 13 May 1888—21 February 1993 (Royal Society memoir). DOI: 10.1098/rsbm.1997.0016
• Intellectually gifted but inherently fragile (2022). DOI: 10.5194/hgss-13-83-2022
• A seismologist's beginnings (2021). DOI: 10.5194/hgss-2021-20
• Thirty-third presentation: William Bowie Medal to Inge Lehmann (1971). DOI: 10.1029/eo052i007p00537
• Inge Lehmann (1888–1993) (AGU notice, 1993). DOI: 10.1029/93eo00411
• A personal memoir of Inge Lehmann (1993). DOI: 10.1029/93eo00520
• Inge Lehmann's work materials and seismological epistolary archive (2009). DOI: 10.4401/ag-4625
• [Comment on "Seismology in the days of old"] The Lehmann discontinuity (1987). DOI: 10.1029/eo068i046p01593-04
• Damage and intensities in the magnitude 7.8 1929 Murchison, New Zealand, earthquake (1994). DOI: 10.5459/bnzsee.27.3.190-204
• P' as read from the records of the earthquake of June 16th 1929 (Lehmann, 1930 — title only). openalex:W572547897
• Observations of the Seismic Shadow Zone from the 29 July 2025 M 8.8 Kamchatka Earthquake. DOI: 10.5194/egusphere-egu26-2627
• How Inge Lehmann Discovered the Inner Core of the Earth (2013). DOI: 10.4169/college.math.j.44.5.399
• The Earth's inner core and the geodynamo (1995). DOI: 10.1029/95eo00143
• Complex inner core of the Earth: The last frontier of global seismology (2015). DOI: 10.1002/2014rg000469
• Danish Seismic Research in Relation to American Nuclear Detection Efforts (2016). DOI: 10.1057/978-1-137-59688-8_8
• Inge Lehmann – seismologiens pioner (2025, in Danish). DOI: 10.7146/kvant.167669
• Lehmann discontinuity beneath North America: No role for seismic anisotropy (2005). DOI: 10.1029/2004gl022333

What came after

• The four layers in our books. The edge of the inner core is one of the three great lines that divide the Earth into crust, mantle, core and inner core. Someone proposed naming it after its finder, as the other two lines (Mohorovičić and Gutenberg) are (DOI: 10.1029/eo068i046p01593-04).
• A name for fairness in science. Lehmann inspired a Danish funding program that aims to make research fairer between women and men (DOI: 10.5194/hgss-13-83-2022).
• An inner core still open. How the inner core changes with direction, how it soaks up waves, the shape of its edge and how it turns are all active topics. What it is made of, how it freezes, and how it helps the magnetic field are still being worked out (DOI: 10.1002/2014rg000469).

Things to try together

• Guess without opening. Hide one thing in a box and let your child shake it, tip it and listen to guess what it is. Ask them to answer, "I can't prove it, but I think it's ○○." This links to the end of the story.
• Count her age. Use her birthday and the day she died to count her age together. It is practice in going back to the first record when numbers disagree.
• Draw the four layers. Draw the Earth cut in half and name the four layers. You might write Lehmann's name next to the last one.