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.

A new study shows that the inner core changed shape in just a few decades, challenging previous assumptions and revealing hidden dynamics deep within Earth.

Scientists found that the inner core’s structure changes as it rotates. It deforms at its border, potentially accumulating more material in some areas and less in others — almost like creating hills and valleys.

The planet’s inner core was previously thought of as a hard, solid sphere. But a new study has found that its edges are softer than realized and actually changing shape.