Discovery
The existence of a solid inner core distinct from the liquid outer core was discovered in 1936 by seismologist Inge Lehmann using observations of earthquake-generated seismic waves that partly reflect from its boundary and can be detected by sensitive seismographs on the Earth's surface. This boundary is referred as Bullen discontinuity or sometimes Lehmann discontinuity.
Later (1940) it was hypothesized that this inner core was solid iron, and its rigidity was confirmed in 1971.
The outer core was believed to be liquid due to its inability to transmit elastic shear waves; only compressional waves are observed to pass through it. The solidity of the inner core has been difficult to establish because the elastic shear waves that are expected to pass through it are very weak and difficult to detect because they also must travel through the outer core. Dziewonski and Gilbert established the consistency of this hypothesis using normal modes of vibration of Earth caused by large earthquakes. Recent claims of detections of inner core transmitted shear waves were initially controversial but are now gaining acceptance.
Read more about this topic: Inner Core
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—Henry David Thoreau (1817–1862)