The Great Flood was an event in human history when the Archangel Michael slaughtered legions of humans on God's orders. Once humans started to populate the earth, they became degenerate and began to worship angels instead of God. Because of this, God ordered Michael to wipe out mankind. A man named Noah built a bunker to shield him and other humans from Michael.
Over time, humans developed symbols to explain this story, and after many years the story evolved. When it was written down, people took the symbols literally, and wrote down that God sent a flood to wipe out humans, and Noah built an ark to protect him and the earth's animals.