deep-ocean

The compound "deep-ocean" is a modern English formation that transparently combines two ancient roots to specify a particular marine environment. The first element, "deep," originates from a Proto-Indo-European root meaning hollow or deep, a concept retained through its Germanic lineage into Old English. The second element, "ocean," has a grand mythological origin in Greek cosmology, where Ōkeanós represented the world-encircling waters, later adopted into Latin and Romance languages before entering English. The compound's logic is directly descriptive, fusing the concept of great vertical extent with the vast expanse of the sea to denote the profound, lightless regions far from the continental shelves and surface waters.

Structure: deep (having great depth) + ocean (the vast body of salt water) - deep: From Old English dēop (deep, profound, serious), from Proto-Germanic deupaz, from Proto-Indo-European dʰewb- (deep, hollow). Functions as an adjective describing extent downward. - ocean: From Old French occean, from Latin ōceanus, from Greek Ōkeanós, the name of the great river or sea believed to encircle the Earth's disc. Functions as a noun specifying the large body of water.