A mysterious layer at Earth's core may be fueled by water

Space · Published 31 October 2026 Swipe more like this →

In short: Deep beneath our feet, a thin but enigmatic boundary exists between the Earth's inner and outer core. For decades, scientists struggled to explain its origin—theorizing everything from ancient magma to lunar collisions. New research suggests the answer might be as simple as a steady trickle of water.

The layer in question is known as the E' layer (or E-prime). Located approximately 1,800 miles (2,900 km) beneath the surface, it sits at the boundary where the liquid outer core meets the solid inner core. While its exact composition was long debated, some scientists initially suspected it was a remnant of iron-rich magma or debris left behind from a collision with a protoplanet that helped form the moon. Geologists first identified this thin zone during the 1990s. It is described as a swirling ocean of liquid metal. This specific boundary marks where seismic waves begin to behave differently.

A new study published in Nature Geoscience suggests a different mechanism: 'leaking water.' As tectonic plates subduct into the Earth's interior, they carry water down toward the outer core. When this water reaches the metallic heart of the planet under extreme pressure, it reacts with the liquid metal. Specifically, hydrogen from the water replaces silica within the metal, forcing the silica out as crystals. These reactions produce large quantities of silica crystals as byproducts. These minerals are then fed into the mantle above. The study used laboratory experiments to replicate these high-pressure conditions.

This process creates a layer that is likely rich in hydrogen and depleted of silica. Exceeding 100 kilometres (60 miles) in depth is the E' layer. Because it likely took over 1 billion years to reach this size, researchers believe it may actually be older than the inner core itself, which solidified roughly 1 billion years ago. This discovery fundamentally changes our understanding of how the mantle and outer core interact. The layer is relatively slim compared to other sections of the interior. This finding suggests that our current knowledge regarding core interactions remains incomplete.

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