Google doodle celebrates the discover of the Earth outer/inner core. Video.

Inge Lehmann ForMemRS (May 13, 1888 – February 21, 1993) was a Danish seismologist and geophysicist who postulated the Earth's inner core.

In 1936, she theorized from existing seismic data the existence of an inner core with physical properties distinct from the outer core's and that Earth's core is not a single molten sphere. 

Seismologists, who had not been able to propose a workable hypothesis for the observation that the P-wave created by earthquakes slowed down when it reached certain areas of the inner Earth, quickly accepted her conclusion.

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