The internal or deep water cycle controls the volume of the oceans at the surface of the Earth. The advent of subduction 2–3 billion years ago initiated the transport of water back to the Earth's interior. With one ocean mass injected into the deep mantle over the last 2–3 billion years, some mantle regions must have become saturated and thus turned into a deep source of water. The mantle transition zone (MTZ) between 410 and 660 km depths is unlikely to be a source of hydrous melt, because its minerals can integrate several thousand ppm of water. On the contrary, the low-velocity layer (LVL) lying above the 410 km-discontinuity is one such source. As proposed by the “Transition-Zone-Water-Filter Model”, the LVL is ubiquitously formed by the global uplift of the hydrous MTZ as a counter flow of subduction of slabs into deeper regions. The seismic signature of the LVL is compatible with the presence of 0.5–1% melt. This melt is produced by dehydration melting during upwelling of the mantle transition zone (MTZ) containing 2200(300) ppm wt H2O, which corresponds to 0.6 ocean mass stored today in the MTZ. Hydrous silicate melt can be gravitationally stable just above the 410 km discontinuity. We propose, here, that at the upper limit of the LVL it becomes buoyant, especially where the mantle is particularly hot and/or hydrous. Once it becomes buoyant, the melt can percolate rapidly upwards through the mantle. As a consequence, the olivine-bearing mantle (OBM) could be almost saturated in water, due to the presence of upwelling hydrous melt. On its path, the melt may be responsible for the seismic low-velocity zones at mantle depths of between 80 and 150 km. It could also be a source for refertilisation of the lithospheric mantle. Based on this model, there should be ~1.0 oceanic mass (OM) stored in the upper mantle today. Secular cooling of the mantle implies an increased capacity of the OBM minerals to store water. The related decrease of oceans' mass at the Earth's surface is estimated to ~20% per one billion years.