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. Nammu is the goddess who had the idea of creating mankind as assistance for the gods.
After the sumerian mythology, around year 4000 BC (date agreed by most of specialists and begining of the sumerian area), a sumerian couple lived in Dilmun (identified today as the isle of Bahrain) : Nammu and her husband An.
After her death, Nammu became the Goddess of the sea (abzu) and An became the God of the Sky. Nammu was the mother and ancestor of all the sumerian gods. Nammu will be later called : Ningal and An : Nanna (their Akkadian names).
Nammu, in later mythology, is depicted only as the primordial sea and loses the humanlike characteristics she may once have had.
In this conceptualization, Nammu (the primordial sea) is "the bitter waters." Notice how she provides the groundwork for the "sweet water of the Apsû," upon which the "earth of living humans" is supported that, in turn, rests beneath the sky (atmosphere) with "an/amû" (heaven) above.
For this to be so, Nammu must be an outwardly rough 'character.' Her exterior must be as rough and bitter as her world was in order that she may thrive in it.