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.

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.