The highest-resolution map yet of the underlying geology beneath Earth’s Southern Hemisphere revealed something we had never known before: an ancient ocean floor that may wrap around the core.

This thin yet dense layer lies at a depth of about 2,900 kilometers (1,800 miles) below the surface, according to a study published in 2023.

That depth is where the molten, metallic outer core meets the rocky mantle above it. This is the core-mantle boundary.

“Seismic investigations, such as ours, provide the highest resolution imaging of the interior structure of our planet,” said geologist Samantha Hansen from the University of Alabama when the results were announced.

“We are finding that this structure is vastly more complicated than once thought.”

Researchers in cold-weather gear unload a large orange equipment case from a plane onto a snowy Antarctic field site.
Researchers lower seismic equipment into place at one of the stations as part of research into the Transantarctic Mountains. (Lindsey Kenyon)

Understanding exactly what’s beneath our feet – in as much detail as possible – is vital for studying everything from volcanic eruptions to the variations in Earth’s magnetic field, which protects us from the solar radiation in space.

Hansen and her colleagues used 15 monitoring stations buried in Antarctic ice to map seismic waves from earthquakes over three years.

The way those waves move and bounce reveals the composition of the material inside Earth.

Because the sound waves move more slowly in these areas, they’re called ultralow velocity zones (ULVZs).

Mantle convection
Rock movements in the mantle. (Hansen et al., Science Advances, 2023)

“Analyzing [thousands] of seismic recordings from Antarctica, our high-definition imaging method found thin anomalous zones of material at the core-mantle boundary everywhere we probed,” said geophysicist Edward Garnero from Arizona State University.

“The material’s thickness varies from a few kilometers to [tens] of kilometers. This suggests we are seeing mountains on the core, in some places up to five times taller than Mt. Everest.”

According to the researchers, these ULVZs are most likely oceanic crust that has been buried for millions of years.

Earth core and mantle
Seismic waves from earthquakes in the southern hemisphere were used to sample the ULVZ structure along the Earth’s core-mantle boundary. (Edward Garnero and Mingming Li/Arizona State University)

The sunken crust isn’t near recognized subduction zones on the surface – zones where shifting tectonic plates push the rock down into Earth’s interior.

But simulations reported in the study show how convection currents could have moved the ancient ocean floor to its current resting place.

It’s tricky to make assumptions about rock types and movement based on seismic wave movement, and the researchers aren’t ruling out other options.

However, the ocean floor hypothesis seems the most likely explanation for these ULVZs right now.

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There’s also the suggestion that this ancient ocean crust could be wrapped around the entire core. Though, as it’s so thin, it’s hard to know for sure. Future seismic surveys should be able to add further to the overall picture.

One way the discovery can help geologists is by figuring out how heat from the hotter, denser core escapes into the mantle.

The differences in composition between these two layers are greater than those between the solid-surface rock and the air above it in the part we live on.

“Our research provides important connections between shallow and deep Earth structure and the overall processes driving our planet,” said Hansen.

The research has been published in Science Advances.

An earlier version of this article was published in April 2023.