The Abzu or Apsu (Cuneiform: ? ?, ZU.AB; Sumerian: abzu; Akkadian: apsû, ), also called engur (Cuneiform:?, LAGAB×HAL; Sumerian: engur; Akkadian: engurru--lit., ab='water' zu='deep'), is the name for fresh water from underground aquifers which was given a religious fertilising quality in Sumerian and Akkadian mythology. Lakes, springs, rivers, wells, and other sources of fresh water were thought to draw their water from the abzu. In this respect, in Sumerian and Akkadian mythology it referred to the primeval sea below the void space of the underworld (Kur) and the earth (Ma) above.
In the city of Eridu, Enki's temple was known as E2-abzu (house of the deep waters) and was located at the edge of a swamp, an abzu. Certain tanks of holy water in Babylonian and Assyrian temple courtyards were also called abzu (apsû). Typical in religious washing, these tanks were similar to Judaism's mikvot, the washing pools of Islamic mosques, or the baptismal font in Christian churches.
The Sumerian god Enki (Ea in the Akkadian language) was believed to have lived in the abzu since before human beings were created. His wife Damgalnuna, his mother Nammu, his advisor Isimud and a variety of subservient creatures, such as the gatekeeper Lahmu, also lived in the abzu.
Abzu (apsû) is depicted as a deity only in the Babylonian creation epic, the Enûma Elish, taken from the library of Assurbanipal (c. 630 BCE) but which is about 500 years older. In this story, he was a primal being made of fresh water and a lover to another primal deity, Tiamat, a creature of salt water. The Enuma Elish begins: "When above the heavens (e-nu-ma e-li?) did not yet exist nor the earth below, Apsu the freshwater ocean was there, the first, the begetter, and Tiamat, the saltwater sea, she who bore them all; they were still mixing their waters, and no pasture land had yet been formed, nor even a reed marsh." This resulted in the birth of the younger gods, who later murder Apsu in order to usurp his lordship of the universe. Enraged, Tiamat gives birth to the first dragons, filling their bodies with "venom instead of blood", and made war upon her treacherous children, only to be slain by Marduk, the god of Storms, who then forms the heavens and earth from her corpse.