The Earth’s inner core seems to hold a deeper secret.

Geology books inevitably include a cut-out chart of the Earth showing four neatly defined layers: a thin outer shell of rock on which we live known as the crust; the mantle, where rocks flow like a highly viscous liquid driving the movement of continents and the lifting of mountains; a liquid iron-nickel outer core that generates the planet’s magnetic field; and a solid inner core.

Two Australian scientists, analyzing the intersection of seismic waves from large earthquakes, said that there is a completely different layer in the center of the Earth. “We have now confirmed the existence of the innermost inner core,” said one of the scientists, Hrvoj Tcalcic, professor of geophysics at the Australian National University in Canberra.

Dr. Tkalcik and Tan Soon Pham, a postdoctoral researcher, estimate that the innermost, inner core is about 800 miles across. The entire inner core is about 1,500 miles across. Their findings were Published Tuesday in Nature Communications.

While the cut-out diagram appears to depict clear divisions, knowledge about the depth of the Earth is inescapably murky. It is about 4,000 miles from the center of the Earth, and it is impossible to drill more than a few miles into the Earth’s crust. Most of what is known about what lies beneath comes from seismic waves—the vibrations of earthquakes that travel across and around the planet. Think of them as a giant sonic image of the Earth.

Two Harvard seismologists, Miyake Ishii and Adam Dezyonski, first He proposed the idea of ​​a deeper inner core in 2002 Based on the velocity characteristics of seismic waves passing through the inner core. Scientists already knew that the speed of seismic waves traveling through this part of the Earth varies with direction. Waves moved faster when traveling from one pole to the other along the Earth’s axis and slower when traveling perpendicular to the axis. Geophysicists believe that the difference in velocities – a few percent faster along the polar tracks – arises from the alignment of iron crystals in the inner core.

But in a small area in the center, the slowest waves were those moving at an angle of 45 degrees to the axis rather than 90 degrees, Harvard seismologists said.

There was too little data available at the time to convince everyone.

The best measurements would be seismic waves traveling from the earthquake’s origin directly to the Earth and through the deeper inner core. However, detecting this generally requires a seismometer located almost exactly on the other side of the Earth, and that point is in the middle of the ocean.

The new paper takes advantage of the fact that seismic waves also bounce back. Thus, a seismometer close to the epicenter can detect the reflection of a wave that has passed through the Earth and bounces back, passing the innermost inner core twice. It can also be reflected back and forth a second time, traveling through the inner core four times.

In recent years, a large number of seismographs have been deployed, especially in the United States. The combination of signals from multiple instruments made it possible to detect faint reflections from earthquakes of magnitude 6 or greater. “We processed 200 events and found that 16 of them had these feedback waves,” said Dr. Tkalcic.

In one of the earthquakes that erupted in the Solomon Islands in 2017, waves traveling five times through the innermost core were detected by seismometers serendipitously placed on the other side of the planet.

“Kudos to them for revealing observations that further studies may use to unravel the confusion of the structure of the inner core,” said Georg Helfrich of the Institute of Earth and Life Sciences of the Tokyo Institute of Technology in Japan who was not involved in the research.

There does not appear to be any great difference in composition between the outer and innermost parts of the inner core, and the transition appears gradual rather than sharp.

This may indicate some change in Earth’s ancient past, said Vernon Cormier, a University of Connecticut physics professor who was not involved in the research. The inner core is fairly young, geologically speaking, Dr. Cormier said — estimates range from 600 million to a billion years old. This is just a fraction of the planet’s 4.5 billion year history, and the structure of the solid core appears to be complex. In January, other scientists reported that the inner core’s rotation speed was changing.

“The reason people study the internal infrastructure is because they are trying to connect it to the Earth’s magnetic field,” said Dr. Cormier. “People will try to look for some changes in the Earth’s magnetic field that may have occurred at the same time as the change in crystallization of the inner core.”