Deep under the Pacific Ocean, between 900 and 1,200 km deep, scientists have discovered something that doesn’t fit with what textbooks say about our planet. We are not talking about something small, but huge areas of rock in the lower mantle where seismic waves move faster than expected.
There is a major problem with this: they are located under open ocean and continental interiors where there is no geological record of subduction in the last 200 million years. Basically, there is no evidence that plates dove down into the mantle in those places. So, let’s learn more about this serious mystery for geophysicists.
How scientists ‘’see’’ under the Pacific Ocean
Nobody can drill 1,000 km under the Earth’s surface. So, how were scientists able to discover these structures? The answer is simple: earthquakes.
Every time an earthquake happens, it sends waves traveling through the entire planet. These waves change speed depending on the material they pass through. Scientists measure when the waves arrive and how they bend or bounce using instruments called seismographs. It works like a medical ultrasound, but on a planetary scale.
For decades, most global images of our planet’s mantle were created by measuring only certain types of waves, mainly P waves and S waves. This helped scientists build the first three-dimensional maps of the mantle. However, this method had limits because it worked best in areas with many earthquakes and many instruments, such as around the Pacific Rim. So, large regions under older ocean plates or quiet continents remained unclear or blurry.
New technology changing the picture
The new study, made by researchers from the ETH Zurich and their collaborators, used a different technique called “full waveform inversion.” This method studies the entire earthquake signal, not just selected parts.
By including reflected and refracted waves that were usually ignored, the researchers improved the detail of their model, even in areas with fewer earthquakes or fewer instruments. Then, to process this enormous amount of data, they used the Piz Daint in Switzerland. With it, they built a high-resolution global seismic model called REVEAL.
What the new model shows
The REVEAL model shows that our planet’s mantle is way more heterogeneous than what it was thought. In the middle and lower mantle, researchers identified many large regions where seismic waves move faster than average.
Normally, faster seismic waves mean the rock is cooler, denser, or chemically different. Traditionally, geophysicists interpreted these fast zones as “slabs” — the cold remains of oceanic plates that sank into the mantle at subduction zones and continued descending over millions of years.
However, the new findings challenge that view. When scientists compared the anomalies detected with detailed reconstructions of past plate boundaries, they found that only about 60% to 70% of known subduction zones aligned with the fast-wave regions. When they corrected for sampling bias, the statistical connection largely disappeared.
So, what are they then?
Lead author Thomas Schouten explains the situation clearly: with the new model, anomalies appear “everywhere in our planet’s mantle,” yet researchers still do not know exactly what material is creating them. The study suggests multiple possible explanations:
- Some structures may indeed be remnants of old tectonic plates.
- Others could be very ancient mantle material rich in silica that formed about four billion years ago and somehow survived Earth’s internal mixing.
- Some may be areas where iron-rich rocks have slowly accumulated as mantle currents moved material around for billions of years.
Previous geochemical and geodynamic research has long suggested that the mantle is not perfectly blended. Instead, it may resemble a “marble cake” made of different rock types mixed unevenly. The new seismic images support this idea and show that differences in composition can look similar to temperature changes in wave speed data.
To sum up
Although these structures lie far beneath the Pacific Ocean, their importance is not distant from daily life. The mantle’s slow movement drives tectonic plate motion, feeds volcanoes, influences long-term sea level changes, and plays a role in the carbon cycle between our planet’s interior and the atmosphere.