Whether we fly over it, sail across it, or simply stand on a shore to stare at it, the ocean appears immense. We must remember that what we are seeing is not really the ocean, but merely the entrance to it. When we pass through that entrance, something remarkable happens. The moment water covers our mouth and nostrils, our heart rate slows. It is as if our bodies remember that millions of years ago life began in the ocean, and now if we return to it we will need to conserve oxygen. This response to being submerged is known as the diving reflex, and it is found in all mammals and exhibited strongly in aquatic ones, such as seals, otters and whales. But any welcome that we humans experience in our return to water is short-lived.
How Deep is the Ocean?
If we hold our breath and attempt to swim downward, we feel a force pushing us back to the surface. The Archimedes Principle, supposedly discovered when the Greek mathematician lowered himself into his bath and noticed the water rising as he submerged, says an upward or buoyant force is exerted on a body immersed in fluid equal to the weight of the fluid that the body displaces.
Should we devise a way to overcome our positive buoyancy and continue our descent, we are unable to breathe. Despite our slower heart rate reducing the oxygen supplied to our body cells, we will die in a few minutes when the involuntary action of gasping will fill our larynx with water and deprive our lungs of air. If we attempt to breathe air from the surface through a tube, we immediately discover it is impossible to inhale. Once our bodies are fully submerged the tiny rib muscles are not strong enough to push out against the weight of the water.
Diving, even to twenty feet (6 m), we experience excruciating pain in the ears. The weight of the water pushes on the outside of our eardrums, which are thin flaps of skin, stretched tight, in our ear canals. Unless we somehow relieve the pressure, our eardrums will burst inward, leaving us partially deaf. When this happens, we will be about thirty-three feet (10 m) underwater and the pressure on our bodies is twice what they are used to. Doubling the pressure on our lungs means they will be compressed to half their usual size. The air in them will be at twice the pressure we normally breathe. That air consists of approximately twenty percent oxygen and eighty percent nitrogen. If we continue to go deeper, breathing compressed nitrogen will have adverse physiological effects on us. Somewhere in the vicinity of 100 feet (30 m) we will begin to experience nitrogen narcosis. It affects our judgement, and unless we return to the surface, we risk hallucinating or doing something irrational, such as attempting to breathe like fish.
What is the average depth of the Ocean?
Meanwhile our lungs, now under an additional three atmospheres of pressure, have compressed further. If we somehow contrive to breathe compressed air to equalise the weight of the water, the nitrogen in that air will become liquid and be forced into our body tissue. If we stay at 100 feet, this will continue to happen until our body tissue is saturated with nitrogen. If we return to the surface too quickly, the compressed nitrogen, suddenly relieved of the pressure, will bubble out into our bloodstream. The bubbles will congregate in our joints and cripple us, or reach our heart and kill us. All this happens if we manage to reach 100 feet in an ocean with an average depth of 12,000 feet (3658 m) and is, in places, more than 35,000 feet, or seven miles (10.7 km), deep.
We can, of course, invent a vessel to protect us from the water pressure and seal ourselves inside it. Knowing, as we do now, that we inhale oxygen and exhale carbon dioxide, we will need to invent a way to supply our bodies with oxygen, and remove the carbon dioxide within our sealed vessel. And it will need to be incredibly strong. As we go deeper, the water pressure will increase until, at the bottom of the ocean, our protective shell will have more than seven tons pressing on each square inch of its surface, or more than a metric tonne on each square centimetre. The smallest flaw in our engineering will see the vessel crushed like an egg under the wheel of a truck. Any vessel built to withstand such crushing force will, by design, be extremely heavy. If it is dropped in the ocean, with a person inside, it will have no problem sinking to the bottom. Getting it back to the surface will be another matter.
Have people reached the bottom of the Ocean?
Yet over 2,000 years, human beings have continually progressed downward. It started with pearl divers in antiquity, and continued until we reached the deepest part of the ocean in the same decade that we first walked on the moon.
Yet the history of downward discovery—the continual exploring of the Frontier Below—is not as well known as other journeys of exploration. In fact, it is barely known at all. And less is known about what lies beyond the entrance to the ocean, than is known about the surface of Mars.
This ignorance about the depth of the ocean and the challenges that face underwater explorers was highlighted when contact was lost with the Oceangate Titan submersible. The nonsense spoken by the world’s media (not to mention ‘experts’ on social media) would have been comical, if it had not been so tragic.
Chief among the misconceptions was knowledge of the depth of the ocean. People simply did not understand it.
Scientists have traditionally divided the ocean in four zones. (Some now say five.) It is important to realise the diagrams that illustrate the ocean zones in books and web sites are misleading. They are invariably drawn out of proportion. The upper Epipelagic Zone is always illustrated larger to accommodate its enormous variety of life. In reality, it is tiny when compared with the other zones.
Therefore, I offer another way to envision the ocean zones. Imagine you are standing with your feet in the deepest part of the ocean and with the top of your head at sea level. As the tide rises and falls, the difference in water level is roughly equivalent to the thickness of two, or perhaps three, strands of your hair.
Now let’s travel down from the surface. We first enter the Epipelagic Zone, which takes its name from the Greek epi, meaning surface, and pelagos meaning sea. The Epipelagic Zone is also known as the Sunlit Zone because sunlight penetrates the water and brings life to photosynthetic plankton, which converts carbon dioxide into energy. The earth’s rainforests are not, as some people have stated, the ‘lungs of the world’. The Epipelagic Zone is. It produces up to eighty percent of our oxygen. It is also home to ninety percent of ocean life, including the most recognisable forms such as whales, dolphins, fish, sharks and jellyfish.
As we stand in our ocean and continue down, about half way between the top of our skull, and the top of our ears, we leave the Epipelagic Zone and enter the Mesopelagic Zone. This zone, like the others, takes its name from the Greek with meso meaning middle. But we are a long way from the middle, or even the average depth of the ocean. The Mesopelagic Zone is sometimes called the Twilight Zone, because the last faint rays of light from a sun high in the sky, are fading by the time they reach the top of this zone. Vertebrates and invertebrates live here in darkness, with many of them swimming upwards at night to feed. Some plant life also survives here.
On our submerged body, somewhere between the bottom of our nose and the top of our mouth, we leave the Mesopelagic Zone and enter the Bathypelagic Zone. Bathy means deep. This zone is in perpetual darkness. No plant life lives here. Some, water borne organisms are luminescent to attract prey or a mate. Many species here are totally blind, and most live on the detritus that falls from the higher zones.
Just below the bottom of our sternum, before we reach our navel, we enter the Abyssopelagic Zone. Abyss means seemingly bottomless. The water is high in oxygen, but low in nutrients. There is very little discernible life and the water is cold—about 37° F or 3° C. Chemosynthetic bacteria thrive near hydrothermal vents in the Abyssopelagic Zone. What fish and invertebrates do live here feed on these bacteria. This, in a sense, is ground zero in the food chain.
To stand in the deepest part of the ocean we need to stand in one of the trenches between the tectonic plates. In our imaginary exercise, we are standing in the Mariana Trench, located off the Mariana Islands, in the Pacific Ocean. Trenches are extremely narrow. The one we are standing in, begins at our groin. In the 1950s, scientists began to notice distinct life in the trenches and started referring to them as the Hadal Zone. A Greek derivative again, but whereas the names of the zones above indicate where in the ocean they are located, the Hadal Zone was named to signify what.
Welcome to hell.
This article is an edited extract from the Introduction of The Frontier Below, by Jeff Maynard (Harper & Collins UK, 2024)
