Myths don't have to be possible. That's the beauty of myths.
Stories of a great deluge occur widely in mythology throughout the world. The biblical story of Noah is a version of a flood myth that originated in Mesopotamia and was familiar throughout the ancient Near East from the time of the Sumerians (around 2500-2000 BCE). All the known versions of this myth have the same basic outline: a deity or deities send a deluge to destroy the world, but one righteous man is fore-warned of the disaster and builds a ship in which he and his family survive the flood. After the flood has subsided, the world continues to exist and is repopulated.
Before God sends the flood, the Bible tells us that human beings "began to multiply on the face of the ground" (Genesis 6:1). A similar expression occurs in the flood story in the Mesopotamian epic of Atrahasis, where the reason for the deluge is the gods' desire to curb human overpopulation.
There are striking resemblances between the Genesis story and the account of a great flood related by Utnapishtim, who is the equivalent of Noah, in the Babylonian epic of Gilgamesh. For example, Utnapishtim survives the flood by building a ship. The vessel finally comes to rest on Mount Nisir, which is in the same region as "the mountains of Ararat", where Noah's ark comes to rest (Genesis 8:4). Utnapishtim opens a window of his ship and sends out a dove, a swallow and a raven to reconnoitre the ground, much as Noah opens the window of the ark and sends out a raven and a dove.
On emerging from the ship, Utnapishtim offers a sacrifice which propitiates the gods when they smell its sweet savour, and they agree that in future humanity should be punished if it behaves wickedly, but not destroyed. Noah likewise offers a sacrifice, and after smelling its "pleasing odour", Yahweh promises: "I will never again curse the ground because of humankind ... nor will I destroy every living creature as I have done" (Genesis 8:21). Finally, both Utnapishtim and Noah are blessed and rewarded. Utnapishtim is granted immortality, while Noah lives for another three hundred and fifty years and is granted a fecund progeny that will repopulate the world.
Similar stories include the ancient Greek myth of Deucalion, and the Hindu story of Manu.
Guide To The Bible by J R Porter, Professor Emeritus of Theology, University of Exeter, apart from first and last paras.