Water in the Mantle: A Hidden Ocean Beneath Our Feet express
Deep beneath us, hundreds of kilometers down, lies a layer that can absorb water like a sponge. The minerals there, wadsleyite and ringwoodite, can hold up to 1–2% water by weight within their crystal lattices. That means the mantle transition zone (between 410 and 660 km) could potentially hide more water than all the oceans on the surface combined. But is this sponge actually soaked?
The authors of a new study built a computer model to trace the fate of water in this layer. They combined the physics of the solid mantle, mineral chemistry, and heat flow. The model showed that water entering the transition zone doesn't just sit there as dead weight. It makes the rock lighter, causing it to rise — slowly, at centimeters per year, but relentlessly. Over tens of millions of years, this 'wet' upwelling reaches the boundary where wadsleyite transforms into olivine, and there the water is released.
The released water lowers the melting point of the rock, forming melt droplets. These are lighter than the surrounding mantle, so they continue upward — now faster, seeping through solid grains or rising as separate 'bubbles.' A similar process occurs at the lower boundary of the transition zone: cold slabs (subducting oceanic plates) descend from above, carrying water as well. At depths around 660 km, they heat up, water is expelled, and light melts start their journey back up.
Thus, the transition zone turns out to be not a storage room but a relay point. Water lingers there for 80 to 430 million years, and at any given moment, there's not much of it — less than 0.1% by weight, meaning less than a fifth of the mass of today's oceans. But that's enough to regulate the water level at the surface. If oceans shallow, the mantle releases some moisture through volcanoes; if there's too much water, some sinks back down with the plates. Such a thermostat system might also work on other rocky planets, helping them keep their oceans for billions of years.
The mantle transition zone contains minerals that, at pressures of 130,000 atmospheres (like the weight of 1,300 elephants per square centimeter), rearrange their crystal lattice and can absorb water — in amounts comparable to the volume of several oceans.
In Jules Verne's novel 'Journey to the Center of the Earth,' the heroes find an underground ocean — our model shows that water really can hide in the mantle, but not as a sea, rather inside crystals.