By some accounts there is more water trapped in the mantel than there is above the crust.
I believe that’s true.
Many, many minerals have a “hydrated” form where there are crystalline forms based mostly on the base mineral, but structurally bonded with water molecules.
Concrete is a good example. When you mix it, you use water, and it’s not diarrhea loose, but it’s pretty fluid.
As it sets, the water does not evaporate away, it gets bonded into the carbonate crystals.
Just like limestone.