Mysterious formations discovered deep inside Earth

Six vast structures that had never been recorded before are hiding about 2,900 kilometers beneath Earth’s surface, where the mantle meets the liquid outer core. Chinese researchers detected them through faint seismic signals, with help from an artificial-intelligence model.

The boundary lies in an environment of extreme heat and pressure. Viscous mantle rock meets liquid iron and nickel there, while temperatures change by roughly 1,000 degrees Celsius. The newly identified structures are effectively invisible, but they subtly alter the way seismic waves travel through the deep Earth.

In a study published in JGR Solid Earth, the researchers said the formations could be remnants of material that moved from the planet’s upper layers into its interior over immense geological periods. Some may contain pieces of continental crust. Another possibility is that part of the material came from Theia, a hypothetical Mars-sized protoplanet believed to have struck the young Earth about 4.5 billion years ago. That collision is thought to have contributed to the formation of the Moon.

Material buried at such depths would not remain unchanged. Under pressures and temperatures reaching thousands of degrees, rocks can partly melt, change their mineral composition and undergo complex chemical transformations. The researchers suggest these processes may have produced “thermochemical piles” that differ from the surrounding mantle.

The team focused on unusual signals known as PKP precursors. These weak waves arrive at seismic instruments shortly before stronger earthquake waves and scatter off small differences in the composition and structure of deep layers. Because the signals are difficult to identify among thousands of yearly seismic records, researchers trained an AI system on signals previously labeled by experts. It analyzed more than two million records from approximately 5,000 earthquakes and found 174,929 high-quality PKP signals, more than ten times the number included in all earlier research.