Apsu

In addition to heaven, earth, sea, and underworld, Mesopotamian cosmology believes in an intermediate region between the earth and the underworld, belonging to the god Ea.

Though not entirely consistent in mythologies, and the relationship between the Apsu and the sea and the Apsu and the underworld is ambiguous, Horowitz (Mesopotamia Cosmic Geography, 344) claims that the Apsu has common elements in most accounts: 

“The
Apsu is always filled with waters, the Apsu is always lower than the earth’s surface,
and waters identified with Apsu are found below the surface of rivers,
seas, and other bodies of water. For example, groundwaters in the Apsu are
located beneath the earth’s surface, and Apsu waters in marshes, swamps, seas,
and rivers are lower than the banks of these bodies of water. These common
elements suggest that the Apsu can be thought of as a cosmic subterranean lake
that maintains a constant surface level. If so, it would be logical for the Apsu to
form groundwaters below the earth’s crust, where’the earth’s surface is higher
than the Apsu, but also possible for Apsu waters to seep into marshes, swamps,
rivers, and the ocean, when the earth’s crust dips below the level of the Apsu.
Other deeper portions of the Apsu, of course, could be located beneath the hard
floors of these bodies of water, just as groundwaters flow below the courses of
rivers and canals.”

Some myths ignore this region altogether: “Inanna and Istar reach the
underworld in lnanna’s Descent and The Descent of Istar without ever crossing
the Apsu, and the lands float on a raft above the waters of the tamtu ‘sea’,
instead of the Apsu, in The Bilingual Creation of the World by Marduk.”

Apsu waters can spring up and engulf the land, and they do unless there is a stopper on the top of the bottle, usually in the form of a temple/mountain that keeps the chaos at bay. When the waters of the Apsu are filtered through the temple, they lose their wild danger and become life-giving.