If the water volume deep in the mantle is the same as all the water in the ocean, there are 332,500,000 cubic miles of water in the mantle.
The surface area of the earth is 197,000,000 sq mi. A simple division shows that the water in the ocean would rise by 1.6 miles if all the mantle water were added to it.
The tallest mountain is over 4 miles high.
That means the mantle water will not cover all the mountains of the earth. You would need added ocean water to to at least 3 times more to cover all the mountains. Even then, how would you get 3 oceans full of water to drain off in a few months?
Creationism needs a whole lot of 'splainin to do.
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