“In literature and magic spells, Mesopotamians referred to the beginnings of the world. The various accounts were quite different, so there was no one story that should be considered standard or universally accepted.
The most comprehensive and ambitious of the accounts of creation, contained in the Babylonian Epic of Creation, refers to a time before the present generation of gods in which there were two waters, male and female, commingling with no distinct separation. Rather than the ‘chaos’ which it is often considered, this combination was dynamic and productive, yielding several pairs of older gods, male and female, and ultimately a much larger number of younger gods, on whom the narrative focuses.
The successive generations of gods were progressively stronger and greater than the preceding ones, suggesting that the Mesopotamians visualized a dynamic past development of their universe, culminating in a present order that was stable and unchanging (Foster 1996: 350):
When on high no name was given to heaven,
Nor below was the netherworld called by name,
Primeval Apsu was their progenitor,
And matrix-Tiamat was she who bore them all,
They were mingling their waters together,
No canebrake was intertwined nor thicket matted close.
When no gods at all had been brought forth,
None called by names, none destinies ordained,
Then were the gods formed within these two.
The Babylonian Epic of Creation ascribed to Marduk, god of Babylon, the reorganization of the cosmos into the form the Babylonians knew it, including the motions of the stars, a firmament above the earth, rivers and mountains of the earth, the phases of the moon, and so forth. Since this account dates only to the second half of the second millennium BCE and presupposes a cosmos centred on Babylon, there is no reason to assign it universal validity in Mesopotamia. The poem appears to contain a variety of earlier materials worked into a coherent narrative.
The universe was visualized as circular, with a firmament below heaven and a corresponding surface above the netherworld. Some speculative texts proposed several layers in the cosmos (Lambert 1975). In any case, heaven was so remote from earth that the land was not visible to a human observer who had risen so high, such as the mythologized king Etana, who flew up to heaven on an eagle, or the sage Adapa, who was summoned to heaven after interfering with the wind.
In later periods, heaven and netherworld were thought to be organized like urban earth, with gates, defences, and large residences for the gods containing reception areas, sleeping quarters, baths and other amenities.”
Benjamin R. Foster, “Cosmology,” John R. Hinnells, ed., A Handbook of Ancient Religions, 2007, pp. 183-4.