A seismologist found Earth has a solid core hiding inside its liquid one

Earthquake waves that should have been blocked by Earth's molten outer core kept turning up at seismic stations where they shouldn't. In 1936, Inge Lehmann worked out that some waves were bouncing off a deeper boundary: a solid inner core nested inside the liquid one. That boundary is now called the Lehmann discontinuity, and no one has ever drilled anywhere near it to check.

— Inge Lehmann, P' — Publications du Bureau Central Séismologique International, 1936