A fortuitous sample from the Transition Zone in the Mantle has demonstrated the long suspected presence of Ringwoodite and its water-rich properties, confirming the idea of buried oceans deep within the Earth. Crustal slabs are believed to carry water into the mantle as they subduct. Such water returns to the crust as water-enabled melts like the granite ‘balloons’ (batholiths) which float up through ‘solid’ rock and lift up the terrain. This new evidence confirms that there’s water down there already, possibly several oceans worth.
The “Nature” paper is: Hydrous mantle transition zone indicated by ringwoodite included within diamond
The SF implications of all that water have not remained unexplored – Stephen Baxter flooded the Earth with mantle water in his tale “Flood” (one-word titles are a favourite of Baxter’s). Of course, Flood-believers posit that at least some of the waters of the Noachic Flood came from the mantle and then returned, but – unlike SF-writers – they’re obliged to explain the mechanism aside from a wave of God’s magic-wand.