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British geologist Richard Oldham [1858-1936] became one of the first seismologists - scientists who use seismic waves to study the earth's interior - when in 1906 he found out that P-waves slowed down at a certain depth.
Furthermore he discovered that S-waves were either reflected back to the surface or disappeared altogether! He explained this by saying that the solid mantle ended at this depth: inside was the liquid core.
In 1909, Yugoslav geologist Andrija Mohorovicic [1857-1936] discovered another boundary, this time much closer to the surface. A marked change in density occurs between the crust and the mantle and this now bears his name: the Mohorovicic discontinuity, or Moho for short!
A famous Danish woman, Inge Lehmann, later provided evidence that the core came in two parts: Oldham's liquid outer core and her own solid inner core. This remains the accepted picture today.
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