Sumerians divided the Universe into four layers: 1) Heaven, 2) the Great Above, 3) the Great Below and the 4) Underworld.
Heaven, ‘An’ in Sumerian, was the top layer of the Universe, situated directly above the Earth. Besides the stars, Heaven contained a city named Anduruna and was the original home of the gods. After the major gods and the 600 lesser gods called the Anunnaki chose to come down to the Earth, Heaven remained the dwelling place of the primeval gods and the 300 lesser gods called the Igigi.
Heaven was regarded as not being far distant from the Earth. Ziggurats, 70-foot stepped pyramids of mud brick, were built to reduce the travel distance for gods going back and forth between Heaven and the Earth. The ziggurats served as stairways from Heaven.
The Great Above
The Great Above included everything between the surface of the Earth and the bottom of Heaven, a domain presided over by Enlil, Lord of Air. Clouds and birds were in the Great Above as were cities and humans.
Most gods lived and built their temples in the Great Above. Courses of both the sun and the moon transversed the Great Above. The Sun God (‘Utu’ in Sumeian, ‘Shamash’ in Akkadian) arose each morning behind from the mountainlands in the East (the Zagros Mountains of Iran), drove the sun across the sky during the day and then, in the evening, went back down under the Earth in the west. The Moon God (‘Nanna Suen’ in Sumerian, ‘Sin’ in Akkadian) sailed the moon across the sky at night, moving also from east to west.
The Earth was a part of the Great Above.
The bottom layer of the Great Above, the Earth, was thought to be square and having four corners. When human mortals were created by Enki, he intentionally designed them to live in the Great Above, where most of the gods chose to live, so they could till the gardens of the gods and produce food to satisfy their hunger and other desires that arose from living on the Earth.
The Great Below
The Great Below included everything beneath the surface of the Earth. Besides the depths of the seas, it consisted principally of two layers: the Abzu and the Netherworld.
The Seas were considered to be the watery remains of the primordial mother goddess, Tiamat, slain by Enlil and interred on the Earth on top of the Abzu. Consisting of saltwater and surrounding the land surface of the Earth, the seas were considered inherently hostile and dangerous to humans.
The Abzu was considered to be the remains of the primordial father god, Abzu, slain by Enki and interred on the Earth. Consisting of sweet water beneath the surface of the ground, Sumerians reasoned the Abzu provided water for the rivers and for all growing plants. Waters of the Abzu were considered inherently good and produced ‘hegal’ (Sumerian for ‘pure good.’) In ancient Sumer, fresh water was considered the primary purifying agent. A human could purify himself by washing with it.
The Netherworld, (Kur in Sumerian, Ersetu in Akkadian), was often referred to by Sumerians .as ‘The Land of No Return.' It was located underground, beneath the Abzu.
In the city of Uruk, there was an entrance named Ganzir that led to stairs that went down into the Netherworld. The Land of No Return could also be reached through some caves in the mountainlands, today's Zagros Mountains on the border between Iraq and Iran. A river named the Hubur flowed into the Netherworld. The principal city was named Irkalla.
Ereshkigal, Inanna’s sister, was Queen of the Netherworld. Her husband, Nergal, served as King. In addition, 600 Anunnaki, lesser gods Enlil had sent down into the Netherworld in the days just before The Flood, lived there and served as judges for the human dead.
In Sumerian myth, the part of the Netherworld where humans were dwelled after they died was dark, dusty and unpleasant. They went about naked, with no water to drink and only dust to eat. However, life for the dead could be ameliorated if the living made timely offerings of food and drink.
The Underworld was the underside of the Earth which was reheated every night when Shamash drove the Sun under it so he could return to the Mountainlands from whence he rose in the morning.
As a result of the sun traveling beneath the Earth, the Underworld became very hot, a place of fire and brimstone, which resulted in naphtha springs and bitumen seeps erupting on the surface of the Earth. Occasionally the constant reheating of the Underworld would turn exceptionally violent and produce volcanoes and earthquakes.