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.

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 located below the middle layer called the mantle and the outer layer –or crust. It consists of two main parts–a liquid outer core and a more solid inner core. The outer layer is made of liquid iron-nickel and is responsible for producing Earth’s magnetic field.