According to ancient Mesopotamian belief, the Abzu (or engur) was the vast freshwater ocean that lay beneath the earth (as opposed to the salt sea, which was thought to surround the earth). The Abzu therefore served as the source of all wells, springs, rivers, streams and lakes.
The Abzu was the domain of the god Enki/Ea, his consort Damgalnuna (Damkina) and his mother Nammu, as well as being the home of a number of his peculiar creatures. (Enki was believed to have ruled the Abzu prior to mankind’s creation). The temple of Enki at Eridu was known as E.ABZU, “the house, or temple, of the Abzu”.
On another level, according to the Babylonian Epic of Creation, Apsû was a primal being, the lover of Tiamat. After Ea’s killing of Apsû, he erected his dwelling on Apsû’s body, the dead creature’s name being transferred thereafter to Ea’s residence. As Ea’s son, the Babylonian chief god Marduk was often designated “firstborn son of the apsû“.
The Underworld was located beneath the Abzu. Occasionally, it seems, the Abzu was identified with a river (the Hubur) that one had to cross in order to reach the Underworld.
The term abzu / apsû was occasionally also used to refer to a tank for holy water in a temple courtyard.