Scientists Uncover Hidden Mega-Structures 100X Taller and Billions of Years Old

When a magnitude 8.0 earthquake rattles a remote corner of the Pacific, the initial jolt lasts only seconds. But the planet does not simply fall silent. For hours afterward, the ground continues to hum at frequencies too low for human ears, the entire Earth oscillating like a struck bell. Thousands of seismometers scattered across continents and ocean floors record these reverberations, capturing data that travels inward as well as outward.

For decades, seismologists have watched certain waves behave strangely after passing through the deep mantle beneath Africa and the central Pacific. The waves slow down dramatically in these regions, as if moving through something fundamentally different from the surrounding rock. The signals are faint, often lost in the noise of the planet’s constant vibrations. But they persist in the data, earthquake after earthquake, pointing to something massive hidden nearly 2,900 kilometers below the surface.

A team of researchers at Utrecht University in the Netherlands spent years compiling these faint signals from the largest earthquakes on record. They focused not on the speed of the waves alone but on how much energy the waves lost along their path, a property called attenuation. By treating the entire Earth as a single vibrating system, they built a model that could finally resolve what lies at the boundary between the core and the mantle.

The Largest Features Inside the Planet

The model revealed two colossal structures that rise from the core-mantle boundary like inverted mountains, reaching heights of roughly 1,000 kilometers. That is nearly 100 times taller than Mount Everest, according to The New York Post. Known as Large Low Shear Velocity Provinces, or LLSVPs, one sits beneath Africa and the other beneath the central Pacific Ocean. Each spans up to 5,000 kilometers across, making them among the largest identified features inside Earth.

The new research, published in Nature, used a technique called normal-mode seismology to analyze the free oscillations of the Earth following powerful earthquakes. Unlike conventional seismic tomography, which maps velocity variations, this method resolves both elastic and anelastic properties. The team built a 3D global model called QS4L3, constraining spherical harmonics up to degree four, the first such model for the whole mantle.

Sujania Talavera-Soza, the study’s lead author, and her colleagues used data from earthquakes strong enough to excite the planet’s normal modes. The method allowed them to distinguish between temperature effects and compositional effects in the mantle, something earlier models could not do with certainty.

In the Nature paper, the authors describe the LLSVPs with a precise qualification: “These are not mountains in the conventional sense,” the authors note, “but thermochemical structures that rise from the core-mantle boundary and influence mantle flow.” Their scale, however, makes them the tallest identified features inside Earth.

A Billion-Year-Old Slab Graveyard

The results showed a striking pattern. In the upper mantle, regions of high attenuation correlate with low seismic velocity, exactly what scientists expect from hot rock. But in the lower mantle, the pattern reversed. The LLSVPs showed low attenuation, meaning seismic waves passed through them more efficiently, despite their low velocity.

That combination points to a distinct composition, not simply higher temperature. The researchers concluded that these structures contain larger mineral grains than the surrounding mantle and are chemically distinct. The team compared their model with wave speeds and attenuation predicted by a laboratory-based viscoelastic model developed by Ulrich Faul of MIT and Ian Jackson. The comparison suggested that the circum-Pacific region is colder with small grain sizes, while the LLSVPs are warmer with larger grain sizes.

The prevailing theory holds that LLSVPs are ancient subducted slabs, oceanic crust that sank into the mantle billions of years ago and accumulated at the core-mantle boundary. Because of their unique chemistry, they resist mixing with the rest of the mantle through mantle convection. “They appear to be chemically distinct domains that have persisted since the early stages of Earth’s history,” the authors write in Nature.

How Normal-Mode Seismology Cracked the Problem

The breakthrough came from the ability to measure seismic attenuation in three dimensions across the entire mantle. Previous global attenuation models only covered the upper mantle. The new QS4L3 model resolves structures down to spherical harmonic degree four, a significant improvement in resolution.

The research team included Laura Cobden of Utrecht University, who contributed to the mineral physics analysis. The team used splitting function measurements from normal modes, analyzing how the frequencies of Earth’s free oscillations shift due to lateral variations in structure. They observed the highest attenuation in the lower mantle in the “ring around the Pacific,” a seismically fast region, and the lowest attenuation within the LLSVPs themselves.

Viscosity calculations performed for the inferred variations in grain size and temperature confirmed that LLSVPs are long-lived, stable features. The results align with earlier studies suggesting that LLSVPs have persisted for hundreds of millions to billions of years.

Anchors That Shape the Surface

The two structures sit directly above the outer core, a region where temperatures rival the surface of the Sun. They are so vast that if either were placed on the surface, its peak would punch through the upper atmosphere and into the blackness of space. Scientists now believe these giants act as anchors, steering the slow, grinding movement of tectonic plates above them and feeding volcanic chains like Hawaii and Iceland with plumes of hot rock rising from their summits.

No human will ever stand on these peaks. No camera will capture their slopes. But every time a major earthquake sends waves ringing through the planet, the echoes carry their shape. The tallest mountains on Earth are not in the Himalayas or even on the seafloor. They are buried more than a thousand miles down, where the mantle meets the core, and they have been there since the planet was young.