Deep within Earth, where hellish temperatures and pressures create crystals and structures like none ever seen on the surface, a strange undulated layer separates the mantle and the core.

The composition of this region, called the d” layer (pronounced “dee double prime”), has puzzled earth scientists ever since its discovery. Now, a team of researchers believes they know what the d” layer is.

Three thousand kilometers deep in Earth, the solid rock of the mantle meets the liquid outer core. At this juncture, seismic waves from earthquakes traveling through Earth suddenly change speed, and sometimes direction.

These sudden shifts trace the border of the d” layer, which rises and falls in ridges and valleys. Researchers suspected that the layer marks a change in the crystal structure of the rock, which might happen at different depths depending on the temperature.

This would explain the…

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