Earth’s mysterious innermost core is a metal ball 400 miles in diameter.

With the exception of the thin crust on which we live, the structure of the Earth is intangible deep under our feet and therefore difficult to imagine.

Food is often used to demonstrate how the Earth is made up of four main layers, a metaphor that is reminiscent of a tasty snack. (will open in a new tab): Graham crackers for the crust, ice cream for the mantle, melted marshmallows for the outer core, and chocolate chips for the inner core.

Scientists have long known about the fifth layer: a 400-mile-wide (650-kilometer) metallic ball inside the inner core, aptly named the innermost inner core. Since it was first theorized (will open in a new tab) in 2002 scientists confirmed (will open in a new tab) his presence several times (will open in a new tab)last time March 2022 (will open in a new tab). However, because it is hidden under the various layers of the Earth and deep inside the planet’s inner core, which itself makes up less than 1% of the Earth’s volume, the innermost inner core is not well understood.

Related: Earth’s inner core may be slowing down compared to the rest of the planet

Now, scientists studying seismic waves generated by large earthquakes have recorded waves bouncing back and forth like a ping-pong ball across the Earth’s diameter five times – the fastest reflection rate ever recorded, beating the previous record by a factor of two. Watching how these waves, generated by the sudden movement of the Earth’s tectonic plates during earthquakes, are distorted as they pass through the center of the Earth, helps scientists better focus on the elusive innermost core.

The team behind the latest study explored the center of the Earth in an innovative way using three earthquake datasets that each saw the core differently, study co-author Hrvoe Tkalcic told in an email. One of the events they studied was a 7.9 magnitude earthquake that hit the Solomon Islands in 2017. (will open in a new tab).

“The earth is shaking like a bell after a massive earthquake, and not just for hours but days,” Australian National University geophysicist and co-author of the latest study, Hrvoe Tkalcic, said in a statement. (will open in a new tab).

To get a good look at the innermost core, scientists need seismometers located at opposite ends of earthquakes — at points they call antipodes — that are often found in the oceans. Thus, they have very little data to work with due to the high cost of setting up seismic stations in such remote areas.

“The innermost inner core is notoriously difficult to explore with seismic waves,” Tkalcic said in an email to

Related: How did Earth’s core stay as hot as the Sun’s surface for billions of years?

So the team combined seismic data recorded by different data centers around the world about the large Solomon Islands earthquake and studied a type of seismic wave called a primary pressure wave or compressional wave. The P wave is the fastest of all seismic waves and the only one that passes through the center of the Earth, so studying its five times crossing the center of the Earth illuminates the interior of the planet.

Tkalcic’s team found that it took 20 minutes for the wave to travel across the width of the planet. Each time this happened, they clearly demonstrated the “anisotropic” property of the innermost core: seismic waves passing through the innermost inner core slowed down in one direction, while waves passing through the outer layer slowed down in the other direction.

“It simply means that the crystals of iron — the iron that is predominant in the inner core — are probably organized differently than those in the outer shell of the inner core,” Tkalcic said in the same statement. (will open in a new tab).

Scientists knew back in 2003. (will open in a new tab) that the innermost inner core is anisotropic (will open in a new tab), so recent research is backing up this knowledge with clearer evidence. In the new study, the researchers found that the P-wave direction inside the innermost core is slowest at an “oblique” angle with the equatorial plane, or 50 degrees from the Earth’s axis of rotation.

“This is very important, and that is why we can say that we found a “distinct” anisotropy in the innermost inner core,” the authors wrote in an article published by The Conversation. (will open in a new tab).

There is strong evidence that slow-moving iron in the Earth’s core is powering the planet’s geodynamo, resulting in the generation of the Earth’s global magnetic field. Thus, understanding what is happening at the very center of the planet will shed light on how the magnetic field behaves, and sometimes vice versa.

While the latest study adds to the growing body of evidence that the innermost core is Earth’s fifth layer, it may be some time before textbooks are updated, Tkalcic told

“After all, when the inner core hypothesis was first put forward in 1936,” Tkalcic said, “it took some time for [the] Earth model to be installed and textbooks to be changed.”

Shortly after the Earth’s inner core hits textbooks, food analogies will follow. Maybe a dark chocolate center inside a chocolate chip?

The study is described in the article (will open in a new tab) published online February 21 in the journal Nature Communications.

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