Inge Lehmann's paper: P'(1936)

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." -Lehmann (1936).In 1936, the Danish seismologist Inge Lehmann , who worked for the Danish Geodetic Institute from 1925 to 1952, suggested from the analysis of P-wave data that the Earth must have an inner core -an important breakthrough in the understanding of the nature of the Earth's interior.