Seismology has identified a specific velocity change under the oceans and continents known as the Mohorovicic Discontuity. It is inferred from the different arrival times of P and S Waves. The velocity differential is explained by assuming rocks above the MOHO are basaltic and those undernearth peridotitic, or a mafic-ultramafic transition.
An alterntaive explanation is that the velocity change is due to water saturated mantle. This interpretation is based on the previous geophysical interpretation of the anticipated rocks during the drilling of the Kola Deep Hole where high velocity seismic records were interpreted as mafic rocks but on drilling were shown to be water saturated gneisses. This was unexpected.
The existence of ‘primary’ water at depth is also inferred from the occurrence of ‘artesian’ water, water that is under subsurface pressure causing it to spurt out from the ground. Artesian water in this sense would then be ‘primary’ water produced by mantle degassing, along with natural gas (methane) which in Australia at least, it is commonly associated.
But mainstream science rejects this explanation preferring instead the recycled water explanation for artesian water, where rainfall in uplands percolates over time downwards into the artesian sedimentary basins.