"Sunken Worlds" Discovered Within Earth's Mantle

There are potential patches of Earth's ancient crust, sometimes called "sunken worlds," that may have just been discovered deep within the mantle, thanks to a new way of mapping the inside of our planet. However, these mysterious blobs appear in places they should not, leaving researchers scratching their heads.

For decades, scientists have been building up a better picture of Earth's interior by using seismographs — 3D images created by measuring how seismic waves from earthquakes reverberate deep within our planet.

The method adopted has helped scientists identify ancient sections of the planet's crust, known as subducted slabs, that have been pulled into the mantle through subduction zones where tectonic plates meet. For example, in October 2024, researchers announced the discovery of a section of seafloor that had sunk deep into the mantle below Easter Island.

In a study published 4 November 2024, in the journal Scientific Reports, researchers revealed that they had discovered "numerous" potential subducted slabs throughout Earth's mantle, using a new type of seismographic imaging.

However, unlike previously identified subducted slabs, which are found in areas where tectonic plates currently collide or have previously smashed together, some of the new anomalies are located in places where no known tectonic activity has ever occurred, such as below the western Pacific Ocean. As a result, it is unclear how they ended up there.

"That's our dilemma," Thomas Schouten, a doctoral candidate at the ETH Zurich Geological Institute in Switzerland, said in a statement released 7 January. "With the new high-resolution model, we can see such anomalies everywhere in the Earth's mantle. But we don't know exactly what they are."

There are other potential explanations for the newly mapped blobs. For example, they may be made of crust-like material left over from the mantle's creation 4 billion years ago. Or they may consist of some other similarly dense material that has grown within the mantle over the past few hundred million years.

However, these are just alternative theories. At the moment, the identity of these blobs remains a "major mystery," ETH Zurich representatives wrote in the statement.

Until now, everything we know about Earth's innards has come from stitching together different seismographs created from different individual earthquakes across the globe. But in the new study, researchers used a new method, known as full-waveform inversion, which uses computer models to combine these seismographs into a single clear image.

This is a computationally intensive method, and to pull it off, researchers had to run the model on the Piz Daint supercomputer at the Swiss National Supercomputer Center in Lugano — formerly Europe's most powerful computer — to crunch the numbers.