The origin of the Earth's oceans has been the subject of much debate. In the last several years the thesis that the oceans derive in a significant part from icy comets has grown in popularity. A recent study suggests that the icy comet thesis is not necessary to explain the Earth's oceans.
The water cycle now appears to involve more than circulation through the atmosphere, oceans, and surface/shallow subsurface waters. A recent study found that as plates subduct, they carry water with them. Geodynamical modeling, high-pressure and high-temperature melting experiments, and analysis of seismic waves passing through the deep earth confirm that a huge amount of water is tied up in the mantle, roughly 300-440 miles below the surface.
This water does not appear to exist in liquid form, but is bound up in minerals that exist at the extreme pressures found at such depths. Interestingly, the finding also suggests that processes that occur in the shallower mantle and that cause volcanoes and related activity at the surface are occurring at greater depths than currently imagined.
The work also supports the idea that the Earth's water accumulated in the interior during its formation rather than arriving through ice comet bombardment. In this new view, water bound up in the mantle degassed over time and reached the surface. The mantle water may also influence the stability or size of the oceans. If the water was not tied up as believed, the oceans would be considerably larger, and there would be little land poking through a huge, planet-wide ocean much deeper than currently exists.
The work helps to connect plate tectonics with the water cycle. They are both part of grander cycle of Earth's processes which help to make and shape our planet.
The study can be found at http://www.sciencemag.org/content/344/6189/1265.