The phrase "water is life" is true – water makes all known life on Earth possible.
It's also a key component in lubricating the super-slow movement of Earth's mantle layer, hydrating those rock layers enough for them to ooze and slide past each other.
This is important to the tectonic cycle, which in turn helps regulate climate over geological time. That water also allows for important recycling of rocks and volatile compounds through the mantle.
"Liquid water is the key component of Earth's habitability," geoscientist Alfred Wilson from the University of Leeds in the UK writes in a commentary accompanying a new study about Earth's interior waters.
How did that water get into the mantle?
One model suggests asteroids brought the water to Earth, and it stayed hydrated as the planet formed. Or maybe the water came later, hydrating a previously dry mantle.
Where exactly that water is currently located within the miles-deep mantle layer of the planet's interior hasn't previously been well understood.
The new research suggests the water is likely located near the boundary between the mantle and its liquid outer core, where seismic tests have shown there are mysterious "ultralow velocity zones."
The lower mantle extends from about 660 to 2,900 kilometers (373–1,802 miles) beneath the surface. Its most abundant minerals, including bridgmanite and ferropericlase, are thought to be largely dry.
Other minerals can hold water at depth, but many either need unusual compositions to remain stable or break down at the high temperatures found in the deepest mantle.
So the researchers went looking for another possibility.
They used laser-heated diamond anvil cells – devices that squeeze tiny samples between two diamond tips just a paper-thickness-width apart, while lasers blast them with heat – to recreate high temperatures and pressures.
Under those conditions, the scientists have identified two previously unknown iron oxyhydroxides (Fe5O12Hx and Fe7O12Hx), that could lock away enormous amounts of water.
The experiments show that these phases can exist under deep-mantle conditions, but do not directly demonstrate that they are present inside Earth.
"Identifying these iron oxyhydroxides is important because they are seemingly stable, dense phases that capture and retain water across a wide range of lower-mantle conditions," writes Wilson in his commentary.
These minerals formed even when water was scarce. In some experiments, the starting material contained less than 0.1 percent water, yet even those trace hydrogen concentrations were enough to stabilize the new phases.
That's important because Earth's deep interior isn't like some giant underground ocean. Any water stored there would have to be incorporated into minerals, often under conditions where free water is essentially absent.
These new minerals appear unusually well suited to the job.
They are both stable at the extreme conditions of the lowermost mantle and substantially denser than surrounding mantle rock.
That means that when a primordial molten "basal magma ocean" cooled and crystallized early in Earth's history, these water-bearing minerals could have formed and then sunk toward the core-mantle boundary.
This hidden water may not necessarily stay hidden, because as water-bearing material is dragged upward by mantle circulation, decreasing pressure could destabilize the minerals, releasing their water into other mantle phases.
Eventually, some of that water could, and probably does, make its way back toward the surface through mantle plumes and volcanism.
The discovery also sheds light on a previous mystery. A mineral known as the "H-phase," observed in earlier high-pressure experiments, appears to match one of the newly identified oxyhydroxides.
The researchers suggest that hydrogen contamination from trace moisture – rather than an entirely new dry mineral – may have helped produce the puzzling phase in earlier experiments.
"Apparently, even very small amounts of hydrogen are sufficient to stabilize these highly hydrated iron compounds," says mineral physicist and crystallographer Leonid Dubrovinsky from the University of Bayreuth.
Related: Colossal 'Anomalies' in Earth's Mantle Aren't What We Thought
There are still big unanswered questions, and the picture is "incomplete" according to Wilson.
Exactly how much water these minerals contain needs to be determined, and what happens when they reach the core-mantle boundary remains uncertain. It is also unclear how easily and over what time periods water stored in these deep minerals can ultimately return to the surface.
Still, the discovery suggests that Earth's water cycle may extend all the way to the edge of the core, and according to Wilson, these newly identified minerals "represent a breakthrough in the mystery of how the Earth obtained and retained its water."
The research is published in Nature Geoscience.
This article was fact-checked by Rebecca Dyer and edited by Rebecca Dyer. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.