At the center of our planet is a very hot rotating ball of mostly iron and nickel (sorry Journey to the Center of the Earth fans). Scientists infer features of this innermost geographic layer—like the fact that it’s divided into a liquid outer core and a solid inner core—by analyzing how it distorts seismic waves caused by earthquakes.

Scientists of course cannot cut into Earth and directly observe its insides. Instead, their knowledge is inferred from the vibrations generated by earthquakes that pass through the planet. The speed and the direction of the seismic vibrations change depending on the density and the elasticity of the rocks.

Earth’s core is transforming, which could affect the length of our 24-hour day, Earth’s magnetic field, and more

Researchers studying decades of earthquake data say they have found the first evidence that, in addition to spinning backward, Earth’s inner core in changing shape.