Nearly 2,900 km beneath our feet, six previously unknown zones have appeared on a new map of Earth's depths.
At this depth, Earth's rocky mantle meets its outer core, which is made of liquid iron. This boundary remains beyond the reach of any drilling operation. To find out what happens there, researchers study vibrations produced by earthquakes and recorded at the surface.
Illustration of a cross-section of Earth, revealing its inner and outer cores.
Image: Argonne National Laboratory / Flickr / CC 2.0
Some vibrations pass through the core before reaching monitoring stations. Along their path, weak signals may appear just before the main wave. They come from vibrations deflected by areas where the rocks differ from their surroundings. These subtle signals therefore provide clues about deeply buried structures.
Finding them is very time-consuming when done manually. Yurui Guan, Juan Li and their colleagues therefore trained a machine-learning system to recognize these signals. They analyzed more than 2 million recordings from earthquakes of magnitude 6 or greater that occurred between 1990 and 2024.
The system identified 174,929 of them, about ten times more than the datasets used previously. This quantity changes the resulting map. Regions that appeared separate in earlier studies now form much more continuous bands in some areas.
To understand what these signals reveal, we need to look just above the core. Researchers already know of regions there where seismic waves slow down sharply. Their composition and origin remain debated. They could contain materials different from the surrounding rocks, or even locally molten sections.
The new analysis detects several of these known regions, making it possible to compare the method with independent observations. Above all, it brings to light six zones with a high probability of scattering seismic waves. They had not been documented before.
These deep differences may preserve traces of our planet's internal history. Some could be linked to ancient pieces of tectonic plates that descended into the mantle or to partially molten rocks. Several mechanisms may also have acted together over time.
The six new zones now provide precise targets for future studies. Researchers will be able to combine several types of seismic waves to obtain more detailed images and test what truly lies at the boundary between the mantle and the core.