Andy Coghlan in New Scientist:
The water is hidden inside a blue rock called ringwoodite that lies 700 kilometres underground in the mantle, the layer of hot rock between Earth's surface and its core.
The huge size of the reservoir throws new light on the origin of Earth's water. Some geologists think water arrived in comets as they struck the planet, but the new discovery supports an alternative idea that the oceans gradually oozed out of the interior of the early Earth.
“It's good evidence the Earth's water came from within,” says Steven Jacobsen of Northwestern University in Evanston, Illinois. The hidden water could also act as a buffer for the oceans on the surface, explaining why they have stayed the same size for millions of years.
Jacobsen's team used 2000 seismometers to study the seismic waves generated by more than 500 earthquakes. These waves move throughout Earth's interior, including the core, and can be detected at the surface. “They make the Earth ring like a bell for days afterwards,” says Jacobsen.
By measuring the speed of the waves at different depths, the team could figure out which types of rocks the waves were passing through. The water layer revealed itself because the waves slowed down, as it takes them longer to get through soggy rock than dry rock.
More here.