Lehmann published her seminal paper 'P′' in September 1936, providing the first evidence that Earth has a solid inner core within a liquid outer core.
In September 1936, Inge Lehmann published her groundbreaking paper titled simply 'P′' in the journal Publications du Bureau Central Séismologique International. In it, she proposed a radical revision to the then-accepted model of Earth's interior: rather than a single molten core, she argued that Earth possessed a solid inner core surrounded by a liquid outer core.
Her insight originated from careful analysis of seismic wave data recorded from a large earthquake near New Zealand in 1929. Lehmann noticed that certain compressional (P) waves, which should have been blocked or refracted away according to existing models of a uniformly liquid core, were nevertheless detected in locations where they should not have appeared. She hypothesized that these waves were reflecting off a solid inner boundary, refracting them back toward the surface in a way consistent only with a solid inner core.
This was a genuine scientific first: no one before her had provided empirical seismological evidence for a solid inner core. Her hypothesis was initially met with skepticism but was confirmed within a few years by other seismologists, including Beno Gutenberg and Charles Francis Richter, using improved instrumentation and theoretical modeling. Lehmann's discovery fundamentally reshaped geophysics and remains foundational to our modern understanding of planetary interiors, achieved at a time when women were rarely permitted to lead independent scientific research.