Apsu in Sumerian-Babylonian folklore, is a primeval abyss of sweet water from which all things sprang...
He is the consort of Tiamat, the primordial abyss of salt waters of Chaos.
In the later legends of the Enuma Elish, the sweet water mingled with the bitter waters of the sea and with a third watery element, perhaps cloud, the first gods were engendered...
The first men were formed by Nammu with clay dug up from Apsu’s waters...
Note that Apsu's vizir, Mummu, was imprisioned in a house built on his body...
"The Epic of Creation (Enuma Elish): Tablet I"
He is the underworld ocean, the begetter of the skies (Anshar) and the earth (Kishar) and the father of Lahmu and Lahamu.
He could not quell the noise of them or of their children, so he colluded with his vizier Mummu to silence the gods and allow his mate Tiamat to rest, after Tiamat herself rejected the idea. Ea found out about his plans, cast a sleeping spell on him and killed him.
(Dalley pp. 232-235, 318)
More:
http://en.wikipedia.org/wiki/Abzu
See Tiamat:
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