"The shark was not an accident. He had come up from deep down in the water as the dark cloud of blood had settled and dispersed in the mile deep sea. . ."
Ernest Hemingway, The Old Man and the Sea.
The waters that essentially cover the earth we inhabit have been a profound source of fascination, imagination, myth and terror since the absolute beginnings of recorded history. Virtually all Polytheistic systems of faith associate one or multiple deities with the oceans: in the Yoruba Religion (similar to Santeria), there is Yemaya, maternal source of life and creation; there's the Roman Neptune, the Greek Poseidon.
The ocean appears again and again in literature as the essential giver of life; as the world's great purifier. Its waves represent birth and death, the unending change of the earth and the universe surrounding it. The kraken emerged from the ocean of man's ancient mind, wrapping unyielding, impossibly giant arms around great travelling vessels and crushing them to vicious splinters. Until recently, the waters have been one of man's great collective canvases, second only to the stars. . .
... It has been in an evolutionary nanosecond that technology has punched holes in the canvas and exposed a wealth of knowledge that, not surprisingly, is even more wondrous than the tales brought forth by the dead civilizations. We have identified the deepest place on Earth: The Mariana Trench, carved at its deepest a full seven miles into the ocean floor. We have discovered vents, spewing chemicals and heat not humanly tolerable from the innards of the planet. Near the vents we found giant tubeworms, fixed to the rocks, thriving on the noxious chemical compounds and boiling heat.
Technically, the region of the ocean known as the "deep sea" begins at a depth of roughly 200 meters (660 feet) and extends downward to the sea floor. Depths between 200 and 3700 meters (12,000 feet) are referred to as steeper continental slope: relatively flat, downsloping underwater land masses extending outward from the continents. The submerged areas of the continents, essentially.
At the end of the continental slope is where the deep ocean basin begins. The floor of the basin resembles that of dry earth--only on a much larger scale. There are mountains and hills and plains and plateaus; the longest mountain range on Earth is beneath the waves. It is the Mid-Ocean Ridge, the place where the world's tektonic plates come apart. It snakes around the globe, 35,000 miles long. (At the equator, the circumference of the earth is approximately 24,000 miles.)
Light from the sun is filtered away long before it has the opportunity to reach the deepest recesses of the ocean (at around 1,000 meters, or 3,300 feet). The pressure in this region is astronomical, able to crush all but the strongest of man's traveling vessels.
The deepest recorded fish was observed at a depth of 27,460 feet.
Many deep sea fish are able to luminate their surroundings with parts of their own bodies, in a process called bioluminescence. Generally, the light serves to attract prey and mates; otherwise, they rely on organic matter drifting from above ("marine snow") for nourishment. Typically, deep sea fish consume their prey (naturally, other, usually carnivorous, deep sea fish) whole and have highly elastic stomachs with which to accomplish this feat. Many of these animals resemble what the people of the dead civilizations may have had in mind when it came to sea monsters.