Scientists at ETH Zurich have solved a geological mystery that has puzzled researchers for around half a century. Deep inside the Earth, seismic waves suddenly accelerate — and the explanation reveals that solid rock thousands of kilometres beneath our feet is slowly flowing.

We tend to imagine the Earth as a series of clearly separated layers: a thin crust, a vast mantle and, deep below, the planet’s core.

But the reality beneath our feet is considerably more dynamic.

At a depth of approximately 2,700 kilometres, close to the boundary between the Earth’s mantle and its outer core, scientists encounter a particularly mysterious region.

It is known as the D″ layer.

For decades, seismic measurements have shown something strange happening there: waves generated by earthquakes can suddenly travel faster.

Researchers have been trying to understand why for roughly 50 years.

Scientists at ETH Zurich (Swiss Federal Institute of Technology Zurich) now believe they have found the explanation.

Using earthquakes to look inside the Earth

No human has ever come remotely close to reaching the D″ layer.

The deepest boreholes extend only a little over 12 kilometres into the Earth’s crust. The region investigated by the ETH Zurich researchers lies more than 200 times deeper.

Scientists therefore have to study the Earth’s interior indirectly.

One of their most powerful tools is the earthquake.

When an earthquake occurs, seismic waves travel through the planet. Their speed and direction change depending on the materials they encounter.

By measuring these waves at different locations on the Earth’s surface, researchers can effectively use earthquakes to investigate structures thousands of kilometres below them.

It is a little like performing an enormous geological scan of the planet.

And those scans revealed the unexplained acceleration of seismic waves near the bottom of the mantle.

A world of extraordinary pressure

Conditions 2,700 kilometres beneath the surface are difficult to imagine.

Temperatures reach thousands of degrees Celsius and pressures are enormous.

Yet much of the mantle remains solid.

That does not mean it is immobile.

Over millions of years, solid mantle rock can deform and flow extremely slowly. This movement is part of mantle convection, the enormous circulation system transporting heat from the Earth’s interior towards its surface.

It is also connected to processes that ultimately influence plate tectonics and volcanic activity.

The researchers suspected that this slow movement might provide the missing explanation for the unusual seismic observations.

The importance of post-perovskite

At the heart of the discovery is a mineral phase known as post-perovskite.

Under the immense pressures and temperatures found near the core-mantle boundary, minerals can reorganise their atomic structures.

Post-perovskite is one such high-pressure structure.

Its discovery two decades ago had already transformed scientists’ understanding of the lowermost mantle. But an important question remained: could the behaviour of its crystals explain the unusual seismic signals observed in the D″ layer?

A team led by Professor Motohiko Murakami, Professor of Experimental Mineral Physics at ETH Zurich, investigated precisely this question.

Recreating the deep Earth in the laboratory

Obviously, the researchers could not simply collect a rock sample from a depth of 2,700 kilometres.

Instead, they recreated aspects of the extreme environment of the deep mantle experimentally and combined their observations with computer modelling.

The experiments allowed the scientists to study how post-perovskite behaves when subjected to deformation under enormous pressure.

Something important happened.

As the material was deformed, its crystals developed a preferred orientation.

Rather than remaining randomly arranged, the crystal structures increasingly aligned themselves in particular directions.

Why seismic waves speed up

That alignment changes the way seismic waves pass through the rock.

Post-perovskite does not transmit seismic waves at exactly the same speed in every direction. Consequently, when many of its crystals become similarly aligned, seismic waves travelling in certain directions can move considerably faster.

This phenomenon is known as seismic anisotropy.

The ETH Zurich experiments showed that the deformation of post-perovskite could produce the type of anisotropy detected by seismologists in the D″ layer.

In other words, the mysterious acceleration of seismic waves may be recording something extraordinary:

the direction in which solid rock is flowing deep inside our planet.

A 50-year-old mystery

Seismologists had known about unusual directional differences in seismic velocities near the core-mantle boundary since the 1970s.

But explaining them physically proved difficult.

The new results provide a mechanism connecting three things that scientists previously struggled to reconcile: the mineral structure of post-perovskite, the deformation of deep-mantle rock and the seismic waves measured at the Earth’s surface.

The discovery therefore does more than explain why certain waves accelerate.

It gives scientists another way of reconstructing the movement of material in a part of the Earth that can never be observed directly.

Solid does not mean stationary

Perhaps the most counterintuitive aspect of the research is that the material being studied is not a liquid.

It is solid rock.

In everyday life, solids appear rigid and immovable. But geological timescales are very different from human timescales.

Under enormous pressure and temperature, and given millions of years, solid minerals can deform and rock can gradually flow.

The Earth’s mantle is therefore not a static shell surrounding the core.

It is an immense, slowly moving system.

Reading the Earth’s hidden circulation

Understanding that circulation is important because the deep mantle is part of the machinery governing the evolution of the entire planet.

Heat escaping from the core and deep mantle drives convection. Material rises and sinks over geological timescales, interacting with the tectonic plates far above.

By learning how minerals become aligned during this process, scientists may be able to use seismic data to reconstruct the direction of mantle flow.

Professor Murakami describes the results as providing a possible way to determine flow patterns at the base of the mantle from observations made at the Earth’s surface.

A planet that is still moving

The research offers a striking reminder that the Earth beneath us is anything but motionless.

Continents move. Tectonic plates disappear into the mantle. Hot material rises from great depths. Minerals change their structure as pressure increases. Even apparently solid rock can slowly flow.

And because nobody can travel thousands of kilometres into the Earth to watch these processes directly, scientists have to decode the traces they leave behind.

In this case, those traces are earthquake waves racing through a layer of rock almost 2,700 kilometres beneath our feet.

What once appeared to be an unexplained change in seismic velocity may therefore be something much more revealing:

a glimpse of the hidden circulation of the planet itself.

Source: Based on “Why seismic waves spontaneously race inside the Earth”, published by ETH Zurich on 5 June 2025. The research was led by Professor Motohiko Murakami of ETH Zurich and published in Communications Earth & Environment.

FUN fact: A famous former student at ETH Zurich was Albert Einstein, who studied there from 1896 to 1900 and later also became a professor at the university.