When was the Earth’s inner core first identified as solid?

Inge Lehmann, a Danish seismologist, identified Earth’s solid inner core in 1936 by analyzing how seismic waves traveled through the planet. She found evidence that waves were being reflected or refracted by a distinct central region. This meant Earth was not just a single core, but a layered one with a liquid outer core surrounding a solid inner core. Her work was a major breakthrough in understanding Earth’s deep interior.