ab = 'water' zû = 'deep', recorded in Greek as Ἀπασών Apasṓn[1]), is the name for fresh water from underground aquifers which was given a religious fertilising quality in ancient near eastern cosmology, including Sumerian and Akkadian mythology.
Thus the divine Mother Earth – on her surface equipped with a bubble of breathable air – was surrounded by Abzû, and her interior harbours the realm of the dead (Irkalla).
The Sumerian god Enki (Ea in the Akkadian language) was believed to have keen eyes and appeared out of the abzû since before human beings were created.
Anchored in the Tablet of Destinies, they founded an organisation to make Mesopotamia fertile through agriculture, but got into a dispute and consequently created the first humans as labour slaves, to peacefully resolve the conflict.
Enraged by the devastation of earth, Tiamat gave birth to monsters whose bodies she filled with "poison instead of blood" and waged war against her traitorous children.