Great deep

The Great Deep is a phrase in the Bible that has two distinct meaning: oceans and subterranean waters. It is best known in reference to the flood of Noah in where it describes the the waters that burst forth as the "springs of the great deep". In this context the great deep describes subterranean waters that flooded the Earth.

Derivation
The phrase great deep is found in four places in the Bible. It is translated from two Hebrew words, (an adjective meaning, much, many, great) and  (a noun meaning, water making noise and translated as deep, depths, deep places, abyss, the deep, sea). Tĕhōm is derived from the root verb meaning to make an uproar or agitate greatly. In the Hebrew language, the phrase became a stereotyped compound noun and therefore always used without the definite article, thus there was not this or that great deep but rather an all encompassing great deep. Franz Delitzsch translated it as unfathomable ocean.

Subterranean waters
The phrase is of great interest to creationists and flood catastrophists because of its use in Genesis 7:11 where the fountains or springs of the great deep are in someway associated with the onset of the global flood.

Creation scientists and Bible scholars conclude that, in this passage, the "great deep" refers to the splitting open of springs of subterranean waters, which along with a torrential downpour caused the worldwide flood to come about. This conclusion can be made from the comparative philology and the Hebrew terminology used in Genesis 7:11 and as well as consideration of the literary structure of 7:11.

Seas and oceans
While making reference to the crossing of the Red Sea by the Israelites, in referring to God, Isaiah exclaims:

Clearly the sea in this passage is "the waters of the great deep". Since the Red Sea is but a part of the all encompassing great deep, i.e. all the seas and oceans, then here is the Bible definition of great deep.

In the two other places where the phrase is also used the same definition is fully satisfactory.

In other words, a judgment by fire will evaporate all the oceans and destroy the land.

In other words, God’s righteousness is as high as the mountains; His justice is as low as the deepest ocean.