Whose observations first showed that earthquake waves can reveal the Earth's core?

By studying how earthquake waves travel through Earth, Beno Gutenberg found that certain waves were absent beyond specific angles from the quake source. This seismic shadow zone indicated a major change in the planet's interior, leading to the identification of the core-mantle boundary at about 2,900 kilometres depth. The discovery showed that earthquakes could illuminate hidden structures deep inside Earth. It also set the stage for later work distinguishing the liquid outer core from the solid inner core. Gutenberg's contribution remains one of the most important in seismology.