The Structure of the Cosmos & Its Waters
Heaven above, the sweet deep below, and the land of the dead beneath that — a cosmos in ordered layers.
Also: cosmos · cosmic geography · apsu · abzu · structure of the universe · heavens and earth · cosmic waters · world structure
Mesopotamians imagined the universe as a set of layers. Above stretched the heavens, home of the high gods and the stars; below lay the flat earth, ringed and undergirded by waters. Beneath the earth's surface ran the Apsu, the sweet groundwater domain of Enki/Ea, and deeper still lay the dark netherworld of the dead. The whole was bounded by cosmic waters and held in its ordered arrangement by the gods who had separated and assigned each region in the beginning. This layered cosmos — heaven, earth, the fresh deep, and the underworld — formed the stage on which all the myths and the life of gods and humans played out.
In short
The Mesopotamian picture of the universe — heavens above, earth as a disc, the freshwater Apsû beneath, the surrounding/cosmic waters, and the netherworld below — a layered cosmos held in order by the gods.
Where it’s found
Reconstructed from the myths broadly (e.g. 'Enki and the world order', ETCSL 1.1.3, in corpus; Enūma Eliš, akkadian-pending) and late scholarly cosmological texts (akkadian-pending; Horowitz, Mesopotamian Cosmic Geography)
How the idea developed across Sumer, Babylon & Assyria
The layered cosmos (heavens / earth / Apsû / netherworld, bounded by waters) is reconstructed from many texts rather than one systematic treatise; details (number of heavens, exact geography) vary and some are known only from late scholarly texts (e.g. KAR 307). Present the broad picture as well-grounded, the fine details as variable.
No single Mesopotamian 'map of the cosmos' survives; the picture is a scholarly reconstruction and its details vary by source.