In Sumerian mythology, Nammu is probably the first of the ancient deities of Sumer — at least in the process of Creation, if not in actual chronology. She was the goddess of the primeval sea that gave rise to heaven and earth, making her also the goddess of creation. She personifies the Apsu, or the sweet fertile waters, as the source of water and therefore fertility. She is both mother of all the gods and wife of An.
In Sumerian mythology, Nammu (more properly Namma http://www.etcsl.org) is the Sumerian creation goddess. If the Babylonian creation myth Enûma Elish is based on a Sumerian myth, which seems likely, Nammu/Namma is the Sumerian goddess of the primeval sea that gave birth to heaven and earth and the first gods. She was probably the first personification of the constellation which the Babylonians later called Tiamat and the Greeks called Cetus and represented the Apsu, the fresh water ocean which the Sumerians believed lay beneath the earth, the source of life-giving water and fertility in a country with almost no rainfall.