The ocean is a magnificent, inhospitable place. Cold, dark, and violent, it is an environment that people have contended with for as long as humanity has existed.
Despite its unforgiving nature, much of our civilization relies on conquering these harsh conditions. Many have traversed the ocean surface, but only relatively recently have we begun to grapple with the final frontier of the sea: the deep.
As you descend deeper and deeper into the ocean, the laws of physics are instantly and harshly working against you. Most of the visible light spectrum is absorbed within 10 meters of the water’s surface, and almost none penetrates below 150 meters, even when the water is very clear.
As you go deeper, the temperature falls, and the pressure quickly becomes immense. Every 10 meters adds another one atmosphere of pressure. Recreational divers can safely descend to 33 meters. On the way down, the divers’ ears have to be equalized constantly.
Slowly but surely, everything becomes more and more blue as the other colors of visible light disappear. Nitrogen narcosis can start to set in, and even at this relatively shallow depth, the surface can feel distressingly far away. It is only safe to stay here for a few minutes.
But far deeper than this, there is work to be done — precise, technical work that requires sharp concentration and hours of manpower.
Working so far below the surface should be outside the realm of human capability, and yet, every day, the ocean floor is occupied by men in bizarre suits, carrying out extremely difficult work.
So how is it, exactly, that we can send people down to these depths to complete complicated tasks, sometimes working underwater for hours at a time? What science is involved in this dangerous, but essential job, that enables these divers to stay alive?
The deep
Commercial divers work to maintain offshore oil rigs and pipelines, completing tasks that require more precision and maneuverability than a remotely operated vehicle can manage. Divers are needed to flip flow valves, bolt pipes together, or clear debris. The work is essentially heavy-duty construction that happens to be under the sea. It’s an isolated and dangerous job and often involves working underwater at depths of up to 500 meters.
There is danger in being so far below the surface, relying on hoses for your air supply, heat, and communications, and dealing with heavy construction materials. But much of the danger the divers deal with does not come from the cold, dark deep itself, but rather, returning from it.
Decompression sickness — or the bends — is a debilitating disorder that happens from a rapid decrease in pressure on the body, causing gasses that were dissolved in tissue to form life threatening bubbles. Any diver has to be very careful to avoid this dangerous phenomenon. But for divers working extremely deep and for long periods, if left unchecked, decompression sickness would be definitively fatal.
Air is made up of roughly 78% nitrogen and 22% oxygen. Normally on the surface, we simply breathe out the nitrogen that we inhale, since our bodies do not use it. But when diving at depth, each breath taken contains many more molecules of oxygen and nitrogen than a breath taken at the surface, due to the increased pressure.
With all of these extra molecules entering the lungs, they begin to accumulate in the body. As the pressure increases, the nitrogen gas enters solution and more and more of it dissolves into the body’s tissues. This dissolved nitrogen is harmless to the human body, if it stays under pressure. But when it’s time to come back to the surface, the problem begins.
As the outside pressure decreases during the diver’s ascent, the accumulated nitrogen forms bubbles in the blood and tissues. As pressure decreases, gas comes out of solution. The process is similar to how, when opening a bottle of soda, the dissolved carbon dioxide in the soda that causes carbonation is turned back into a gas from its liquid state. This causes the fizz heard when the bottle is opened as the gas escapes the bottle.
If the bubbles of gas in an ascending diver’s body are too big, or form too quickly, they can injure tissue, or even block blood vessels. This blood vessel blockage causes pain and, in the worst instances, death.
In regular diving, this risk is mitigated by coming up to the surface gradually, allowing the nitrogen to diffuse slowly out of tissue and be exhaled through the lungs, avoiding the build-up of big nitrogen bubbles.
Diving to 75 meters for an hour, for example, would require a five-hour ascent to avoid getting bent. The longer the dive, the more dissolved nitrogen has built up in the tissue, requiring longer decompression time.
For deep sea divers working at depths much greater than this and for many more hours, the amount of time it would take to safely ascend would be way too long to be feasible.
On top of the deadly effect of decompression sickness, nitrogen plays other tricks on the body. Nitrogen narcosis is a condition that hits many divers when doing deeper dives, usually setting in at around 30 meters.
At this depth, nitrogen narcosis can cause an alteration in consciousness, akin to the feeling of being drunk. It’s usually not harmful in and of itself, but slowed mental activity, giddiness, and overconfidence can lead to divers disregarding safe diving practices.
At 30 meters on a recreational dive, the effects are kind of amusing, and can be simply reversed by ascending a few meters. But as divers go deeper and deeper, the effect can be debilitating and mental impairment may become extremely hazardous.
Below 90 meters, nitrogen narcosis can lead to hallucinations, loss of memory, or unconsciousness, which for deep sea divers working on intricate and dangerous tasks, could quickly become fatal. Scientists don’t fully understand what causes it, but believe nitrogen gas — or any inert gas except helium, and probably neon — react with lipids, or fat tissues in the body which make up most of the brain, causing an anesthetic effect.
Because of the tricky interaction between the physics of gases and physiology of the body, for a long time, deep sea dives remained out of reach. That all changed in the 1960s.
The underwater laboratory
As NASA was launching its effort to put men on the moon, the United States Office of Naval Research was working on their own otherworldly mission: putting men at the bottom of the ocean.
In July 1964, an odd-looking vessel was launched from the Navy’s oceanographic research tower off the island of Bermuda, where it sank to a depth of 60 meters. Twelve hours later, four navy divers entered the SEALAB I, ready to begin a unique 21-day experiment. Their assignment was to participate in the Navy’s first extended physiological test to determine how men can work freely and for long periods deep below the surface.
The primary mission of the SEALAB project was to see if time-wasting, dangerous, daily decompressions while returning to the surface could be eliminated, by providing a shelter near the dive location kept at a pressure equal to the diving pressure. This would, in theory, allow the men to work for longer and at greater depths.
When under pressure, every breath taken contains more molecules of nitrogen and oxygen than on the surface, and the extra nitrogen dissolves in the body’s tissues. But as they found during the SEALAB experiment, after enough time at a certain pressure, the body cannot absorb any more, and becomes fully saturated with it.
More time at that depth will not add any more nitrogen to the tissue and will not add to the length of the decompression time. Because of this, divers can stay pressurized indefinitely, working multiple, long dives, while only needing one long decompression after days, weeks, or even months of time below the surface.
This type of diving was coined “saturation diving,” and is much safer than making multiple short dives that each require their own lengthy decompression. The dives can also be deeper and longer, since decompression can happen in a controlled habitat.
However, while decompression sickness is managed with this method, it does not solve the problem of nitrogen narcosis. Nitrogen breathed at depth would still be incapacitating, whether underwater or in the living quarters.
To avoid this problem, saturation divers don’t breathe normal air. Instead, they breathe a gas cocktail called heliox, which replaces most of the nitrogen in normal air with helium.
Helium does not cause the narcotic effect that nitrogen does, and is harmless to the human body. Decompression from a heliox saturation dive also requires less time than would be required with an air mixture that contains more nitrogen.
However, breathing helium does not come without its own consequences. Because helium is lighter than air, sound waves travel faster through the gas. The helium amplifies the higher frequencies in the voice, creating a silly voice that’s a well-known party trick.
Albeit amusing at parties, after weeks on end in an underwater living situation it can become annoying — and actually problematic. It’s hard to understand divers over communication systems with these voices, so surface personnel often have to use a piece of equipment called a “helium descrambler,” which electronically lowers the pitch of the diver’s voice.
Diving in the modern day
After a series of SEALAB experiments, it soon became apparent that it would be easier and cheaper to monitor and support divers if their pressurized living quarters weren’t actually at the bottom of the sea, but instead, onboard the dive support vessels, and kept at pressure mechanically.
Divers enter the chambers, and the “blowdown” begins. Slowly and carefully, the pressure increases to match the pressure they will experience at working depth. After around 72 hours, the divers’ bodies become saturated with the inert gas.
To get to the seafloor, divers exit their pressure chamber habitat through an airlock, and enter a diving bell, which is also pressurized. The diving bell is then lowered to the required working depth, and the divers exit the diving bell, into the cold dark water to work. Once the divers have finished their shift, they re-enter the bell, which is hoisted back to the surface, and the next shift can begin.
While physically close to others aboard the dive support vessel, the divers may as well be in space. The general rule for desat is 24 hours for each 33 meters of pressure, so it can take days to decompress from a deep dive and rejoin society.
If done carefully, and if there are no catastrophic equipment failures, saturation diving can be done safely. However, the divers have to remain in a pressurized environment for the duration of their work time, which can be as long as three weeks or more. This means living in very close quarters with other divers with no privacy whatsoever. Mentally and physically, it is extremely taxing.
While mostly safe due to advancements in protocols and technology, it is not without its dangers. If an airlock fails, the pressure would explosively decrease, and bubbles would rapidly form in the blood, basically boiling it. The gas in the diver’s body would race out any and all exits. It is immediately, and gruesomely, fatal.
Even with rigorous safety protocols in place, decompression is still hard on the body and comes with a lot of danger. When undergoing desaturation, divers report joint pain, headaches, and shortness of breath.
These symptoms are unfortunately similar to the first symptoms of decompression sickness. Experienced divers know the difference, but if any diver thinks they may be suffering from the bends, the whole team will have to start the decompression again. The only cure for early signs of decompression sickness is to return to higher pressure.
Saturation diving is not for the faint of heart. There is an ever-present sense of danger. Exiting the diving bell, and entering a pitch black underwater world is enough to make anyone squeamish, while the days and weeks on end spent in confined quarters would be enough to make most people go mad.
Carrying out difficult work, while navigating the risks of living in a pressurized environment that could end your life instantly if there is a breach, or the risks as your body becomes depressurized, easily makes this one of the world’s most challenging jobs.
