I am going to assume that you are referring to recent new stories with titles like "Rare Diamond Confirms That Earth's Mantle Holds an Ocean's Worth of Water" (Scientific America).
These articles are referring to research published in Nature: Pearson, D. G. et al. (2014). The researchers found an inclusion of ringwoodite inside a diamond. Here is a quote from the Nature article about the paper...
Using infrared spectroscopy, Pearson’s team found that its tiny fleck
of ringwoodite contained about 1% water by weight. “That may not
sound like much,” Pearson says, “but when you realize how much
ringwoodite there is, the transition zone could hold as much water as
all the Earth’s oceans put together.”
This doesn't mean that there is a liquid ocean of water within the transition zone. There are minerals that are considered "hydrated" because they contain hydroxide ions. Examples of these are the serpentine group and micas.
Ringwoodite can contain hydroxide ions within its structure, and these ions are the "water" that was measured. If you were to lower the pressure, and increase the temperature, the water would be liberated from the mineral. The point is that there are not huge oceans of liquid water within the mantle of the Earth; the water is locked up in the structure of certain minerals.
There are interesting implications from this research discussed in the Nature News article about the research.
Nature News Article