Shadow Zones and Discovery

Seismic waves map Earth's interior through shadow zones. After a quake, P-waves should reach everywhere, yet a ring between 104 and 140 degrees from the epicenter stays mysteriously silent.

Danish seismologist Inge Lehmann cracked the puzzle in 1936. Studying faint waves that did appear inside the shadow zone, she deduced that a solid inner core sits within the liquid outer core — a hidden sphere bending the waves back. She proved Earth has layers without ever drilling more than a few kilometers down.