Earth's inner core
The inner core was discovered by Inge Lehmann in 1929, using seismology. Lehmann was studying a large New Zealand earthquake. An earthquake makes vibrations which move through the inside of the Earth. The vibrations Lehmann studied seemed to be moving across something solid in the center of the planet. She called this the inner core. She wrote about it for many years, but it was not proved to exist until 1970.