Some geological constraints:
The mantle is basically solid. There are small patches here and there of melted rock. Some of the aesthenosphere is about 1% melted. The outer core is melted, but apart from the magnetic field that doesn’t make a noticeable difference for us at the surface.
The water inside the earth is chemically locked in and often isn’t water but hydroxide. An accessible example is plaster. Mix the powder with water and it all turns to plaster (or let it happen naturally and call it gypsum). The water is trapped in the crystal structure. If you heat a bit of plaster or gypsum in an enclosed container, you get steam as the piece turns to powder. But it’s silly to clain that a plaster wall has waters beneath. Bad headlines about there being an ocean in the mantle actually mean that if you could extract all the hydroxide and turn it into water, you’d get a lot of water. Water is the most abundant gas coming out of volcanoes, but as steam.
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