The concept of “water underwater” refers to the immense, hidden reservoirs of water stored within and beneath the Earth’s solid crust, distinct from surface oceans, ice caps, or the atmosphere. This subterranean water is distributed in two primary ways: as liquid water filling porous rock close to the surface and as chemically bound hydrogen and oxygen within high-pressure minerals deep inside the planet. The total volume of this hidden water is vast, potentially rivaling the amount held in all the world’s oceans.
Water Trapped Near the Surface
A significant portion of the Earth’s hidden water is found in the shallow subsurface, stored as groundwater beneath both continents and the ocean floor. This water occupies the tiny spaces, or porosity, within sediments and rock formations. Recent estimates indicate that the total volume of groundwater in the upper crust reaches approximately 44 million cubic kilometers.
This shallow reservoir is composed of two distinct layers. The upper layer, roughly 24 million cubic kilometers, is the usable, mostly freshwater involved in the hydrologic cycle. Deeper down, stretching between two and ten kilometers beneath the surface, lies an additional 20 million cubic kilometers of ancient, highly saline brine. This deep brine is largely isolated from the modern water cycle due to the low permeability of the surrounding rocks.
Beneath the ocean floor, water also saturates the thick layers of sediment and the volcanic rock of the oceanic crust. In some regions, ancient freshwater plumes, known as offshore freshened groundwater, have been discovered extending for dozens of kilometers beneath the seabed. Vast quantities of seawater also circulate through the fractured, porous rock of the sub-seafloor crust, a process that chemically alters the rock and contributes to the planet’s long-term water budget.
The Deep Earth Water Cycle
The most surprising reservoir of water lies hundreds of kilometers beneath the surface within the Earth’s mantle. This water is not liquid, but is integrated into the crystal structure of high-pressure minerals as hydroxyl groups (OH-). This chemically bound water forms the core of the deep water cycle, a process driven by plate tectonics.
The Mantle Transition Zone, a layer between 410 and 660 kilometers deep, is the primary storage region for this deep-earth water. Here, the minerals wadsleyite and ringwoodite, high-pressure forms of olivine, can incorporate significant amounts of water into their structure. Wadsleyite can hold up to 3.3 weight percent water, while ringwoodite can hold up to 1.5 weight percent.
Water is delivered to this depth by the process of subduction, where cold, water-soaked oceanic plates sink into the mantle. Calculations based on the storage capacity of these minerals suggest that the transition zone alone could hold an amount of water equivalent to one to three times the total volume of the surface oceans. The presence of this water influences mantle rock properties, facilitating processes like partial melting and mantle convection, which ultimately links the deep interior back to surface volcanism.