An important discovery A new look at the bottom of the earth from the ocean

Scientists have assembled the most accurate map yet of the underlying geology beneath Earth's southern hemisphere, revealing something never before discovered: an ancient ocean floor possibly coiled around the core.

This thin, dense layer lies about 2,900 kilometers (1,800 miles) below the surface, with the molten metallic outer core and rocky mantle above it. This is the core mantle boundary (CMB). Understanding exactly what lies beneath our feet — in as much detail as possible — is vital to studying everything from volcanic eruptions to variations in Earth's magnetic field, which protects us from solar radiation in space.

"Seismic investigations, such as our study, provide the highest resolution imaging of the internal structure of our planet, and we find that this structure is much more complex than previously thought," says geoscientist Samantha Hansen of the University of Alabama.

Hansen and her colleagues used 15 monitoring stations buried in the ice of Antarctica to map seismic waves from the earthquakes over a period of three years. The way these waves move and bounce off reveals the compositions of materials inside the Earth. Because sound waves move slower in these regions, they are called ultra-low velocity regions (ULVZs).

"By analyzing [thousands of] seismic records from Antarctica, our high-resolution imaging method found thin, irregular regions of material in the CMB everywhere we looked," says geophysicist Edward Garnero of Arizona State University. "Thicknesses of material vary from a few kilometers to tens of kilometers." Kilometers. This indicates that we see mountains on the core, in some places as high as five times Mount Everest."

According to the researchers, this ULVZ is most likely oceanic crust buried over millions of years.

And while the sinking crust is nowhere near recognized subduction zones at the surface — areas where moving tectonic plates push rock down into the Earth — the simulations in the study show how convection currents could shift the ancient ocean floor where it is today. 

It is difficult to make assumptions about the types of rocks and their movement based on the movement of seismic waves, and researchers do not rule out other options. However, the ocean floor hypothesis seems to be the most likely explanation for these ULVZs at the moment.

There is also a suggestion that this ancient oceanic crust could have wrapped around the entire core, although it is so thin it is hard to know for sure. Future seismic surveys should be able to add more to the overall picture.

One way the discovery could help geologists is to figure out how heat escapes from the hotter, denser core into the mantle. The differences in composition between these two layers are greater than they are between the solid surface rock and the air above them in the part where we live.

The research has been published in the journal Science Advances.