Where does new water on earth come from?

New water comes from under the crust of the Earth. It escapes mainly through hydrothermal vents in the sea floor.

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There is a water cycle on earth that takes wet sediments deep into the mantle to be re heated and circulated back up as new sea floor. This is where the water in hydrothermal vents originates - it is NOT new water. However, there remains a possibility that some of the water emerging from the mantle has been there since the formation of the earth so some of it MAY be 'new'.