Well, I do not believe it to be impossible but heres a verse for you, considering your view of impossibility towards the Flood's happening.
Mark 10:27 - Jesus looked at them and said, "With men it is impossible, but not with God; for all things are possible with God."
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Psalm 14:1
The Fool says in his heart, "There is no God."
They are corrupt, their deeds are vile; there is no one who does good.