A massive 'sea' three times the volume of all the planet's oceans has been discovered deep beneath the Earth's surface and close to its core, scientists have said.
The water is hidden inside a blue rock that lies 700 kilometres underground in the mantle, the layer of hot rock between Earth's surface and its core.
The revelation may help explain where the world's seas came from.
Steven Jacobsen of Northwestern University in Evanston, Illinois, told the New Scientist:
It's good evidence the Earth's water came from within.
Scientists believe that water landed with comets as they smashed into the planet.
But the new findings support an alternative theory that the seas have gradually seeped out of the early planet.
The reservoir, within rock called ringwoodite, could also explain why earth has remained the same size for millions of years, by acting as a buffer for surface seas.
– Steven Jacobsen of Northwestern University in Evanston, Illinois
It's rock with water along the boundaries between the grains, almost as if they're sweating.
We should be grateful for this deep reservoir.
If it wasn't there, it would be on the surface of the Earth, and mountain tops would be the only land poking out.
Mr Jacobsen and his team found the water by measuring seismomic waves generated by more than 500 earthquakes.
Until now, the team just had evidence that the watery rock sits below America.