Since humans cannot drill deeper than a few kilometers, we rely on indirect evidence. Seismology acts like an “ultrasound” for the planet. By observing how seismic waves propagate, we discovered that P-waves slow down in liquids and S-waves stop entirely. This revealed that the outer core is liquid. The velocity changes at specific boundaries, like the Mohorovičić and Gutenberg discontinuities, define the transition between the crust, mantle and core. Unlike pressure or temperature data, which are based on models, seismic data provides real-time, physical evidence of the Earth’s complex, layered internal structure.