Nearly 3,000 kilometers beneath Earth’s surface is a puzzling zone known as the D” (D double-prime) layer
For decades, scientists have noticed something unusual here: seismic waves, the vibrations caused by earthquakes, suddenly speed up as they pass through this region. The reason behind this behaviour has remained one of the biggest mysteries in Earth science.
A mineral transformation under pressure
In 2004, researchers identified a major clue. At the extreme pressures and temperatures of the lower mantle, the mineral perovskite, which comprises a significant portion of this region, transforms into a new form known as post-perovskite. This change occurs precisely at the D” layer’s depth and was initially believed to be the reason for the seismic speed-up.
However, further study revealed that the transformation alone wasn’t enough to explain the sudden acceleration of seismic waves.
Crystal alignment and seismic speed
In 2007, advanced computer simulations uncovered a crucial detail: the speed of seismic waves through post-perovskite depends on the direction in which its crystals are aligned. When all the crystals point in the same direction, the mineral becomes stiffer, and seismic waves travel through it more quickly, just like they do in the D“ layer.
This hinted that crystal alignment could be the missing link, but it still needed to be proven in real-world conditions.
To test the theory, researchers at ETH Zurich designed a cutting-edge experiment that simulated the intense heat and pressure found near the Earth’s core. They discovered that post-perovskite crystals do align themselves when subjected to such conditions. In doing so, they reproduced the same seismic wave behaviour observed in the D“ layer, providing the final piece of the puzzle.
The next question was what causes these crystals to align. The answer: horizontal flow of solid rock deep within Earth’s mantle. Though this type of movement had long been suspected, the experiment offered the first direct evidence that solid mantle rock can flow, much like thick syrup, at extreme depths.
This type of flow form of mantle convection, moves heat through Earth’s interior and reshapes how we think about the planet’s deep processes. As it moves, it naturally aligns the post-perovskite crystals, producing the seismic signature scientists have puzzled over for decades.
A dynamic, living planet
This discovery alters our understanding of the Earth’s interior. It confirms that the planet is not only geologically active at the surface, with earthquakes, volcanic eruptions, and shifting tectonic plates but also deep within its mantle. The movement of solid rock at such depths plays a key role in driving these surface events and may even influence the Earth’s magnetic field.
With this new understanding, scientists can begin to map deep mantle currents and gain clearer insights into the invisible engines that power our dynamic planet. What was once hidden beneath nearly 3,000 kilometers of rock is now becoming visible.
![Figure 1: Sketch of the evolution of the Universe over the last 13.77 billion years. It started with the Big Bang, followed by an extremely short period of rapid exponential expansion. The furthest we can see is the cosmic microwave background, when radiation decoupled from matter, approximately 380,000 years after the Big Bang. This is followed by the ‘dark ages,’ during which this radiation redshifted from the visible regime into infrared and sub-mm wavelengths. The occurrence of the first stars, about 400 million years after the Big Bang, ended this phase, spearheading the formation of galaxies as we see them today. [Credit: NASA/WMAP Science Team, public domain]](https://www.openaccessgovernment.org/wp-content/uploads/2025/05/Fig-1_1200-100x70.jpg "How did the first stars form in space?")
