Bold claim first: Earth might hide a colossal, invisible ocean inside its own interior. New findings suggest a vast amount of water could be permanently stored in the lower mantle, locked into minerals like bridgmanite, and still shaping how our planet behaves today. If this holds up, the largest body of water on Earth could be buried about 1,000 miles (1,609 kilometers) beneath our feet, far from the surface oceans we know.
But here’s where it gets controversial: this reinterpretation challenges the long-standing view that Earth’s water came mainly from late delivery by comets or asteroids, and instead supports the idea of a “wet accretion”—water being part of Earth’s building blocks from the very start. Such a shift would ripple through how we understand planetary formation, the history of our climate, and the potential habitability of worlds beyond Earth.
Extreme experiments point to a wetter early Earth
Researchers from the Carnegie Institution for Science, led by Wenhua Lu, used cutting-edge high-pressure, high-temperature techniques to mimic the conditions of the young planet’s interior. They employed a laser-heated diamond anvil cell to reach temperatures above 3,700 Kelvin and pressures over 700,000 atmospheres, approximating the environment inside the lower mantle as the early magma ocean solidified.
Under these extreme conditions, bridgmanite—the dominant mineral in Earth’s deep interior—was found to absorb more water as temperatures rise. The experiments revealed a pattern: water preferentially partitions into bridgmanite at higher temperatures. In other words, a significant portion of water could have remained trapped inside the mantle rather than migrating to the surface.
This insight is echoed by an accompanying Science commentary, which argues that prior models may have drastically underestimated how much water Earth kept during its formative years.
Could a Deep reservoir rival surface oceans?
If these results hold, Earth’s deep mantle might contain water volumes comparable to several of the world’s oceans—though not as liquid water. Instead, hydrogen atoms would be bound within mineral structures, effectively creating a hidden, solid-state ocean inside the planet.
That concept would dramatically expand the scope of the global water cycle, which has traditionally emphasized surface and atmospheric processes. A deep, internal reservoir could help clarify puzzling geochemical signatures observed in mantle plume volcanism, such as in Hawaii and Iceland, where deep-sourced magmas sometimes bear traces of primordial mantle material.
The findings align with growing evidence that Earth’s water doesn’t rely solely on external inputs. Some scientists argue that Earth’s interior has long acted as a water buffer, moderating surface conditions over deep time.
Rethinking the origin of Earth’s water
For decades, the prevailing theory has been late delivery of water by comets or carbonaceous asteroids during a period known as the Late Heavy Bombardment. This study adds weight to the alternative “wet accretion” view, where water is incorporated into Earth’s building blocks from the start and becomes integrated into minerals inside the planet.
If confirmed, this perspective could extend beyond our planet. Many rocky exoplanets may start with internal hydration, harboring hidden water reservoirs even when their surfaces appear dry. For the search for habitable worlds, interior hydration could broaden the criteria beyond surface water signatures alone.
A shifting picture of interior habitability
An enduring, hydrated mantle would not only shape Earth’s early water story but also influence long-term planetary evolution. Internal water can affect plate tectonics, mantle convection, and volcanic chemistry, potentially playing a crucial role in a planet’s thermal and geochemical stability over billions of years.
What comes next
The experimental results are a first step. While we cannot directly observe the lower mantle, indirect clues—from seismic data to xenoliths and geochemical signatures—hint at remnants of this deep water reservoir. As laboratory techniques advance, scientists will map Earth’s interior hydration with increasing precision.
If future work confirms substantial deep mantle water retention, it could transform our understanding of planetary cooling, geodynamo behavior, and long-term climate regulation. This could also reframe how we interpret the history of Earth’s oceans and atmosphere, inviting new debates about how much water is truly stored inside our planet versus on its surface—and what that means for other worlds.
