She was raised in an academic family, with a father who was a psychology professor, but had to navigate numerous obstacles on her path into the world of science. She was modest, encountered resistance due to her gender and struggled with health issues that delayed her career. However, in 1928, at the age of 40, she earned a master’s degree in Geodesy and was employed as a state geodesist at the Geodetic Institute – an unusual achievement for a woman at the time.

By chance, Lehmann developed an interest in seismology and became a key figure in the field’s development in Denmark. Back then, the earth was believed to have a large liquid core surrounded by a solid crust. After analysing data from earthquakes around the world, Lehmann challenged this view.

In 1936, she published her groundbreaking discovery in a research paper titled simply “P′”. Lehmann had found deviations in the seismic P-waves that pass through the earth’s core.

According to the accepted theory, the waves should be stopped or deflected in a certain way by the liquid core, but Lehmann saw that some waves went deeper and were deflected or bent in a way that suggested the presence of a second boundary inside the earth.

She called these non-conforming waves “P′”, and her interpretation of them radically changed the understanding of the earth’s inner structure by suggesting a solid inner core surrounded by a liquid outer core. Today, this inner core, approximately 2,500 kilometres in diameter, plays a key role in our understanding of the earth’s magnetic field.

Despite this outstanding work, it took a long time before Lehmann was recognised for her accomplishments – particularly in Denmark. Other scientists used her work without quoting her, and in 1952 she applied, unsuccessfully, for a Chair of Geophysics at the University of Copenhagen.

After retiring from the Geodetic Institute, in 1953, she spent much of the next 20 years in the United States, continuing her research. Here, her discovery was acknowledged. She published new research findings and received multiple honours, including an honorary doctorate at Columbia University, in 1964.

During the Cold War, seismology rose to prominence as the American government wanted to monitor underground nuclear tests. They established an extensive network of seismographs and needed experienced seismologists, like Lehmann.

In the United Sates, she worked with the geophysicist Maurice Ewing (1906–1974), who was the head of Lamont Geological Observatory. Ewing had met Lehmann a few years earlier, during a visit to Europe, where he was looking for the most talented European seismologists. He convinced the US government that they should bring Lehmann to the United States.

In Denmark, Lehmann received little acknowledgement until 1965, when, at the age of 77, she was finally awarded the gold medal from the Royal Danish Academy of Sciences and Letters – albeit at the suggestion of her colleagues abroad, who wondered why Lehmann was so overlooked in her homeland. Another important milestone occurred in 1971, when Lehmann became the first woman to receive the prestigious William Bowie Medal from the American Geophysical Union.

Her path from the damp bunker under Vestvolden in Rødovre to her role as scientific superstar was long. She did not receive full international and Danish acclaim until after her death, at 104.

Today, Lehmann’s name is commemorated in awards, grants, research programmes, seismic monitoring stations and Danish street names. In 2017, a sculpture was raised in Lehmann’s honour on the central Copenhagen square Frue Plads, next to the bust of Niels Bohr.

The chapter is written by Asser Pelle.