In the early 1900s, scientists were fairly certain that the Earth was made up of many layers, like an onion, but they did not know exactly where the layers started and ended. In 1909 Andrija Mohorovicic (1857-1936), a Croatian seismologist, helped reveal the existence of the shallowest of these layers, the crust, and the underlying layer, the Earth's mantle.

Mohorovicic studied Yugoslavian earthquake records that showed the same set of seismic waves arriving twice on a single earthquake record. Because the second set of waves exactly paralleled the first set, Mohorovicic concluded that the first set had traveled at a faster rate through a denser layer of rock deep inside the Earth. The second set had traveled at a slower rate, and a shorter distance, closer to the surface. This allowed Mohorovicic to interpret the existence of the crust and below it, the mantle. In tribute to...