Bold claim first: Earth hides colossal amounts of water deep underground, reshaping what we thought about our planet’s history—and this discovery could change how we understand Earth’s evolution. But here’s where it gets controversial: the water isn’t just in oceans; it’s embedded in the mantle’s minerals, possibly far more than we imagined.
A new study reports the existence of vast, previously unseen reservoirs of primordial water thousands of kilometers beneath the surface. Using simulations that push to the extreme temperatures and pressures found about 660 kilometers down, researchers found that bridgmanite, the mantle’s most abundant mineral, can retain significant amounts of water even at temperatures around 4,100 C.
To picture the mantle: it’s a thick, semi-solid layer of hot, dense rock between the crust and the outer core. It makes up roughly 84 percent of Earth’s volume and about 67 percent of its mass. The hottest portions move slowly, like viscous plastic, driving long-range convection that influences surface tectonics.
The study, published in Science, suggests that the way water is stored and distributed deep inside Earth may have been crucial in transforming a volcanic, hellish early Earth into the habitable world we know today. Lead author Du Zhixue of the Guangzhou Institute of Geochemistry explains that water could have become “locked away” as molten material cooled and crystallized into solid minerals.
When Earth formed about 4.6 billion years ago, it was nothing like the blue planet we know. Frequent cosmic impacts churned surface and interior into magma, making it too hot for liquid water. Yet, in this fiery youth, substantial water likely became trapped deep inside the planet, Du notes.
As the early magma ocean cooled, it crystallized to form solid minerals, gradually building the mantle. Bridgmanite, the mantle’s first and most abundant mineral, may have acted like a microscopic sponge. Its ability to harbor water could determine how much water stayed trapped from the magma.
By modeling the magma ocean’s crystallization, the team found that bridgmanite’s water-locking capacity could make the lower mantle the largest water reservoir within the mantle once solidification began. Earlier work, based on lower temperatures, suggested bridgmanite held less water. The researchers pushed temperatures up to 4,100 C with a newly developed ultra-high-pressure experimental setup and found that water storage might be five to 100 times greater than previously estimated.
The experiments combined laser heating with high-temperature imaging that mimics early deep-mantle conditions, establishing the temperatures at which water could be stored. This provides a solid basis for understanding how temperature influences water partitioning in the deep Earth.
In collaboration with Long Tao of the Institute of Geology at the Chinese Academy of Geological Sciences, the team used atom probe tomography to examine water distribution at the nanometer scale. Their approach is like giving the microscopic world a high-resolution chemical CT scan, allowing clear visualization of how water is integrated into bridgmanite’s structure.
The researchers estimate that the amount of water retained in the early solid mantle could have been equivalent to about 0.08 to 1 times the volume of all modern oceans.
Water trapped deep underground isn’t a static storehouse. It likely acts as a lubricant for Earth’s enormous geological engine, lowering the melting point and viscosity of mantle rocks and enabling slow, persistent mantle convection. This activity helps drive surface tectonics and sustains the planet’s long-term dynamism.
Over time, this deep water would gradually re-emerge toward the surface through the mantle’s slow movement, contributing to the formation of Earth’s primordial atmosphere and oceans. Water locked within Earth’s early structure may have been a pivotal force in steering the planet from a molten world to the habitable, water-bearing world we inhabit today.
The study received support from multiple Chinese institutions, including the Chinese Academy of Sciences, the National Natural Science Foundation of China, the Ministry of Science and Technology, the Guangdong Basic and Applied Basic Research Foundation, and the China Postdoctoral Science Foundation.
