Seismic Waves Map the Earth's Interior

P waves are longitudinal and pass through solids and liquids alike; S waves are transverse and stop dead at a liquid boundary. Seismometers on the far side of the planet from a large earthquake record a shadow zone in which no S wave ever arrives, and P waves that do arrive are refracted off their expected paths. From those two observations alone — with no sample ever taken — geologists concluded that the outer core is liquid and the inner core is not.