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We might finally know how ocean-sized听deposits听of water hundreds of kilometres below Earth鈥檚 surface are getting there.听A form of clay, called kaolinite, might be soaking up water like a sponge and bringing it deep underground.

Depending on the location, kaolinite听accounts for听between 5 and听60 per cent of ocean sediment . Now, geologists have demonstrated how it can act as an irrigation system for the upper mantle, the mineral and rock layer that descends to more than 400 kilometres beneath our feet.

The clay听gets sucked down underground听when an oceanic plate collides with听the continental crust and nosedives beneath it in a process called subduction, reaching depths听sometimes deeper than听the upper mantle after millions of years.

Lab experiments simulating the escalating pressures and temperatures encountered during the descent have now that kaolinite can absorb huge amounts of water. In its 鈥渟uper-hydrated鈥� form, water accounts for 29 per cent of its weight. This is the maximum amount it can carry, and happens at depths of around 75 kilometres where the clay meets temperatures of 200 degrees Celsius and pressures of 2.7 Giga Pascal, more than 25,000 times the pressure at sea level.

Halfway to the upper mantle boundary, at depths of around 200 kilometres where the temperatures top 500 degrees Celsius and pressures听exceed 5 Giga Pascal, the clay starts to lose its water, and continues doing so till it reaches the boundary and beyond, down as far as 480 kilometres.

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As it escapes, the water mixes with surrounding mantle rock and lowers its melting point, turning it to magma. 鈥淲hen the mantle rocks melt, magma is generated and it can lead to volcanic activity when the magma rises to the surface,鈥� says of the Yonsei University in Seoul, South Korea, and head of the team.

The huge water content is roughly twice the 14 per cent found in serpentine, previously the most water-rich mineral widely subducted into the mantle, according to听 of the University of Bristol, who was not part of the research team. 鈥淭he researchers may have opened up a whole new potential water recycling route on Earth,鈥� he says.

鈥淭his study will force us to re-evaluate how much water survives the trip down into the mantle, and suggests larger magma volumes are being produced beneath volcanic arcs than previously thought,鈥澨齭ays听 of Northwestern University in Evanston, Illinois.