About 4.6 billion years ago, the young Earth was a hostile and violent place. Repeated collisions with space rocks left the planet covered in a global ocean of molten rock.
Temperatures were so extreme that liquid water could not exist on the surface.
Yet today, oceans cover about 70% of Earth, raising a long-standing question: how did our planet manage to keep its water through such a fiery beginning?
A new study published in Science offers a powerful answer.
An international research team led by Professor Du Zhixue from the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences has found that vast amounts of water could have been locked deep inside Earth’s mantle as the planet cooled and solidified.
This hidden water, stored far below the surface, may have played a crucial role in Earth’s transformation into a habitable world.
The key lies in a mineral called bridgmanite. Although unfamiliar to most people, bridgmanite is the most abundant mineral inside Earth, making up much of the lower mantle at depths greater than 660 kilometers.
The researchers discovered that bridgmanite can trap water within its crystal structure, acting like countless microscopic water containers deep underground.
Earlier experiments had suggested that bridgmanite could only store small amounts of water. But those studies were conducted at relatively low temperatures and did not fully reflect the extreme conditions of early Earth. To overcome this limitation, the team developed advanced laboratory techniques to recreate the intense pressures and temperatures found deep within the mantle during the planet’s molten phase.
Using a diamond anvil cell combined with laser heating, the researchers raised temperatures to more than 4,100 degrees Celsius, closely mimicking the conditions of Earth’s early magma ocean. They also employed cutting-edge tools capable of detecting tiny traces of water in samples smaller than a human hair. These techniques allowed the team to directly observe how water is stored inside bridgmanite at the atomic level.
The results were striking. The researchers found that bridgmanite’s ability to hold water increases dramatically at higher temperatures.
During Earth’s early, ultra-hot stage, this meant that large amounts of water could be captured and retained in the deep mantle as molten rock slowly crystallized. This finding overturns the long-held assumption that the lower mantle is nearly dry.
Computer models based on the new data suggest that after the magma ocean solidified, the lower mantle became the planet’s largest water reservoir.
Its storage capacity may have been five to 100 times greater than previous estimates. In total, the amount of water trapped in the solid mantle could have ranged from a small fraction to as much as the volume of all Earth’s modern oceans.
This buried water did not remain locked away forever. Instead, it helped shape Earth’s long-term evolution. Water deep in the mantle lowers the melting temperature of rocks and makes them flow more easily, supporting mantle circulation and plate tectonics.
Over billions of years, some of this water was slowly released back to the surface through volcanic activity, contributing to the formation of the atmosphere and oceans.
According to the researchers, this hidden “spark of water” inside Earth may have been the key factor that allowed the planet to cool, stabilize, and eventually support life.
By revealing how Earth preserved its water through its most extreme early phase, the study provides new insight into why our planet became blue and alive—while others did not.
