About 3,200 kilometers below the surface lies a relatively thin but distinctly dense layer at the core-mantle boundary (CMB).

What observations show

After analyzing thousands of seismic recordings from Antarctica, our high-resolution imaging method revealed subtle regions of anomalous material everywhere we examined at CMB.
— says Edward Garnero, a professor in the School of Earth and Space Studies at Arizona State University, co-author of the study.

The team tracked resonant seismic waves propagated during earthquakes, which they noticed slowed significantly in parts of the CMB. These decelerating zones are known as ultra-low velocity zones (ULVZ), which look like mountains protruding from the CMB.

The thickness of the material there varies from a few kilometers to tens of kilometers. This indicates that in the core we sometimes see mountains five times higher than Everest. Based on the fact that seismic waves are geologically slowed in such a thin layer, the researchers hypothesize that ULVZs must be composed of materials from the ancient ocean floor that were much denser than the surrounding mantle.

These mountains are free from insignificant irregularities. Researchers believe they are could play a crucial role in determining how heat escapes from the Earth’s molten core. It is also likely that this oceanic material will eventually be gradually brought to the surface by volcanic eruptions and reused to form the floor of present-day oceans.