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Possible remnants of Earth’s ancient crust, often referred to as “sunken worlds,” may have recently been identified deep within the mantle, due to an innovative approach to mapping the interior of our planet. However, these enigmatic formations seem to appear in regions where they ought not to be, leading scientists to ponder.
For many years, researchers have been constructing a clearer depiction of Earth’s interior by utilizing seismographs—three-dimensional visuals generated by evaluating how seismic waves from earthquakes propagate deep within the planet. This technique has enabled scientists to locate aged segments of the planet’s crust, termed subducted slabs, which have descended into the mantle through subduction zones where tectonic plates converge. For instance, in October 2024, investigators announced the identification of a piece of seafloor that had descended deep into the mantle beneath Easter Island.
In a paper published on Nov. 4, 2024, in the journal Scientific Reports, researchers disclosed that they had detected “numerous” potential subducted slabs scattered throughout Earth’s mantle, employing a novel type of seismographic imaging. (Limited details regarding the size, shape, and precise locations of the formations have been disclosed to date.)
Nevertheless, differing from previously identified subducted slabs that are situated in areas where tectonic plates are currently colliding or have once collided, some of the recent anomalies are found in regions devoid of any known tectonic activity, such as beneath the western Pacific Ocean. Consequently, it remains uncertain how they came to be there.
“That’s our conundrum,” Thomas Schouten, a PhD candidate at the ETH Zurich Geological Institute in Switzerland, stated in a statement issued on Jan. 7. “With the newly enhanced resolution model, we are seeing such anomalies distributed throughout the Earth’s mantle. However, we are uncertain regarding their true nature.”
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There are other possible interpretations for the recently mapped formations. They could be comprised of crust-like substances that originated from the mantle’s formation 4 billion years ago. Alternatively, they might consist of other similarly dense materials that have developed within the mantle over the last few hundred million years.
However, these remain merely alternative hypotheses. Currently, the true identity of these formations is still a “significant enigma,” as mentioned by representatives from ETH Zurich in the statement.
Identifying “sunken worlds”
Up until now, our understanding of Earth’s depths has been derived from piecing together various seismographs created from different individual earthquakes around the world. However, in this recent study, researchers applied a new method, referred to as full-waveform inversion, which utilizes computer models to amalgamate these seismographs into a unified clear image.
This method demands considerable computational power, and to execute it, researchers utilized the Piz Daint supercomputer at the Swiss National Supercomputer Center in Lugano—previously the most powerful computer in Europe—to process the calculations.
Study co-author Andreas Fichtner, a seismologist at ETH Zurich who developed the full-waveform model utilized in this new research, likened the implementation of full-waveform inversion to advancements in medical imaging. Imagine a physician who has been investigating the circulatory system for years, Fichtner stated. “Then, if you provide [them] with a superior examination tool, [they] suddenly discover an artery in an unexpected place,” Fichtner elaborated. “That’s precisely the sentiment we have regarding these new findings.”
Researchers believe the newly identified formations might be subducted slabs, largely since seismic waves travel through them at similar speeds. Nevertheless, this does not confirm their equivalency, and further investigation is necessary to determine whether they truly share the same characteristics.
“We need to compute the diverse material parameters that could account for the observed speeds of the different types of waves,” Schouten indicated. “Fundamentally, we must delve more deeply into the material properties that influence wave speed.”
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