Deep within Earth’s core, scientists are uncovering signs of a hidden structure that challenges long-standing models of the planet’s interior.
Far below the surface, beyond the reach of drilling, Earth’s core may contain a previously unknown layer.
Scientists now believe Earth’s center is not a single uniform sphere but a more complex structure, with a distinct zone buried inside the solid inner core. Often referred to as the “innermost inner core,” this region is reshaping how researchers understand the planet’s deepest interior and may hold clues to how Earth formed billions of years ago.
Earth is traditionally divided into four layers: crust, mantle, outer core, and inner core. “Traditionally we’ve been taught the Earth has four main layers,” said lead author Joanne Stephenson, a PhD researcher at The Australian National University. “The idea of another distinct layer was proposed a couple of decades ago, but the data has been very unclear.”
Probing the Inner Core With Seismic Waves
The inner core is a dense, solid sphere made mostly of iron and nickel, with temperatures exceeding 5,000 degrees Celsius (9,000 degrees Fahrenheit). Although it accounts for only about 1 percent of Earth’s volume, it provides important insights into the planet’s history. Because it cannot be directly observed, scientists study it using seismic waves from earthquakes. These waves travel at different speeds depending on the materials they pass through.
The ANU team analyzed decades of seismic data using an advanced algorithm that tested thousands of models. Instead of averaging the data, which can hide subtle features, they examined the wave patterns more closely. This approach revealed a shift in seismic behavior about 650 kilometers (roughly 400 miles) from Earth’s center, suggesting a boundary within the inner core.
Evidence for a Distinct Central Region
One important clue comes from anisotropy, meaning seismic waves move at different speeds depending on their direction. In many models, waves travel faster along Earth’s rotational axis than along the equator. In this newly identified central region, that pattern shifts.
The slowest wave speeds occur at an angle of about 54 degrees relative to the rotation axis, indicating a different alignment of iron crystals.
