concept

Seismic shadow zone

A band of a planet's surface that some class of seismic wave simply never reaches after a quake. Earth has two, and both carry information. P waves go missing between roughly 103° and 143° from the epicentre, because entering the slower core refracts them sharply elsewhere. S waves fare far worse: beyond 103° they never return at any distance, since every path there must cross the liquid outer core. The beauty of the method is that the measurement is the *absence* of a signal rather than any signal at all.