The water is thought to reach such depths when tectonic plates, containing all the components of the Earth's crusts, get pushed beneath one another so that the lower plate descends into the mantle where the intense heat melts it. From there the components are subject to mixing processes we don't yet fully understood.
Besides being another example of how amazing our world is, the work is relevant to a very old debate: Where do Earths' oceans and atmosphere come from. Although water and various gases were present in the early Solar System, the bombardment Earth faced early in its existence from large objects would have seared the planet's surface.
This has led to two theories. One holds that water and certain gases survived within the mantle, and were released through volcanic eruptions. The other suggests comets and icy meteorites delivered fresh material to the planet once conditions had calmed enough for them to remain.
The debate has a long history – even Newton expressed the view that the atmosphere was of cometary origin long before we knew how Earth formed. The discovery of how deep water can go might appear to support the mantle as a likely store of water, but Kendrick says it is the opposite.
His work adds credibility to a recent study that found evidence the certain gases in the mantle got there by being absorbed from the atmosphere. This research suggests they were initially of cometary origin, rather than having been an original part of the mantle.