How did scientists find out about earth's core has two parts?

Scientists discovered that Earth's core has two parts—a solid inner core and a liquid outer core—through the study of seismic waves generated by earthquakes. By analyzing how these waves travel through the Earth, they observed that certain waves could pass through the outer core but were reflected by the inner core, indicating different states of matter. Additionally, the behavior of these waves provided insights into the core's composition and physical properties. This research was pivotal in understanding the Earth's internal structure.