Chinese scientists have discovered six previously unknown structures sitting 1,800 miles (2,900km) beneath the surface on the boundary of the Earth's mantle and core.
The formations are known as deep-seated scatterers. They are almost invisible, but researchers can locate them by the way they change the path of seismic waves passing through the planet.
A study published in the journal JGR Solid Earth suggests the structures formed when material from outer layers was dragged deep underground.
The researchers say the material could include pieces of continental crust, or remains of the Mars-sized protoplanet Theia. Theia is believed to have crashed into Earth 4.5 billion years ago to form the moon.
Under immense pressure and temperatures at the mantle boundary, chunks of rock transform, partially melt or undergo mineral changes. The study says this process has left six thermochemical piles of material that is very different from the surrounding mantle.
Violent boundary
Nobody has ever journeyed to the centre of the Earth, but physicists have worked out its likely structure by studying shockwaves from earthquakes.
The Earth is made up of four major layers. The crust is the rocky outer shell where life exists, measuring between 3 and 43 miles thick. Below that is the mantle, which consists of hot rock measuring about 1,802 miles thick and makes up 84 per cent of the volume of the planet.
The outer core is a fluid layer of iron, nickel and small quantities of other metals, which is a 1,242-mile (2,000km) thick layer heated to 5,500 degrees Celsius (9,932 degrees Fahrenheit). At the centre is the inner core, a solid ball of iron about the size of the moon where temperatures reach 5,700 degrees Celsius, which is as hot as the surface of the Sun. Crushing pressure caused by gravity prevents the inner core from becoming liquid.
The boundary between the mantle and the outer core is a violent and dynamic place, where solid but viscous rock meets the liquid core. The temperature jumps by about 1,000 degrees Celsius (1,800 degrees Fahrenheit) across the boundary.
Massive heat escaping from the outer core drives convection currents in the mantle known as mantle plumes. These currents play a key role in deciding where volcanic activity occurs on the surface.
Because the layers have hugely different densities, seismic waves slow down dramatically when they cross the boundary, allowing geologists to map what is happening.
To investigate the hidden structures, researchers looked at PKP precursors. These are relatively weak seismic waves that arrive just before the stronger waves triggered by earthquakes.
PKP precursors scatter when they hit hidden structures, known as heterogeneities, on the mantle boundary. They pass through the liquid outer core, but not the solid inner core, and bounce back to seismic detectors ahead of the main wave.
Artificial intelligence
These precursor waves are so weak that they are extremely difficult to find in seismic data, and traditionally had to be searched for manually among thousands of signals collected every year.
The scientists wrote in their paper: "Manual identification of these precursors is inefficient, subjective, and insufficient for vast global seismic data sets."
To solve the problem, the researchers trained an artificial intelligence model to spot PKP precursors. After training it on waves that humans had already identified, they let the system analyse over two million recordings from 5,000 different earthquakes.
The model identified 174,929 high-quality PKP precursor signals, which is more than ten times the number found in all previous studies combined.
Continuous belts
The findings give geologists an unprecedented view of the mantle boundary and reveal huge areas of structures that were previously unknown.
The researchers said: "We also discovered six areas that likely host significant heterogeneities that had never been documented before, providing clear priority targets for future exploration of Earth's deep interior."
Earlier studies had found fragmented, seemingly random structures in a few places around the world. The new map shows those fragments are connected into much larger continuous belts.
The researchers are not sure exactly what the structures are made of or how they formed, but say they are different from the surrounding mantle. They believe artificial intelligence models will help clarify the picture soon.
They wrote: "As the catalogue continues to expand, its high-resolution spatiotemporal coverage will advance the refinement of fine-scale structural models of the lowermost mantle, and offer increasingly rich constraints for deepening our understanding of the geodynamic state of Earth's deep interior."
Magnetic field
The outer core where these structures are located is responsible for generating the magnetic field that engulfs our planet.
Differences in temperature, pressure and composition in the outer core cause convection currents in the molten metal as cool, dense matter sinks and warm matter rises. The Coriolis force, which is caused by the spin of the Earth, also creates swirling whirlpools.
This flow of liquid iron generates electric currents, which in turn create magnetic fields. Charged metals passing through these fields go on to create electric currents of their own, creating a self-sustaining loop known as the geodynamo.
The spiralling caused by the Coriolis force means the separate magnetic fields are roughly aligned in the same direction, combining to produce one vast magnetic field.
