It was Lehmann who, in 1936, first put forward the view that the Earth's core consisted of two parts – an inner and an outer, separated by a discontinuity. For many years it had been thought that the Earth consisted merely of a core, mantle, and crust that were separated by the discontinuities discovered by Beno Gutenberg and Andrija Mohorovičić. This was partly based on the realization that the primary (P) waves of an earthquake are not detected in a shadow zone between 105° and 145° from the epicenter. The reason for this was their diffraction by the Earth's core.

Lehmann found that P-waves increased their velocity quite sharply within the core. She reasoned from this that there is an outer core and an inner core separated by a further discontinuity about 700 miles (1200 km) from the center of the Earth and sufficient to bend some of the P-waves into the shadow zone.