Scientists call it the mammalian dive reflex or, more lyrically, the Master Switch of Life, and they've been researching it for the past fifty years. The term Master Switch of Life was coined by physiologist Per Scholander in 1963. It refers to a variety of physiological reflexes in the brain, lungs, and heart, among other organs, that are triggered the second we put our faces in water. The deeper we dive, the more pronounced the reflexes become, eventually spurring a physical transformation that protects our organs from imploding under the immense underwater pressure and turns us into efficient deep-sea-diving animals. Freedivers can anticipate these switches and exploit them to dive deeper and longer.
At sea level, we are ourselves. Blood flows from the heart to the organs and extremities. The lungs take in air and expel carbon dioxide. Synapses in the brain fire at a frequency of around eight cycles per second. The heart pumps between sixty and a hundred times per minute. We see, touch, feel, taste, and smell. Our bodies are acclimatized to living here, at or above the water's surface. At sixty feet down, we are not quite ourselves. The heart beats at half its normal rate. Blood starts rushing from the extremities toward the more critical areas of the body's core. The lungs shrink to a third of their usual size. The senses numb, and synapses slow. The brain enters a heavily meditative state. Most humans can make it to this depth and feel these changes within their bodies. Some choose to dive deeper. At three hundred feet, we are profoundly changed. The pressure at these depths is ten times that of the surface. The organs collapse. The heart beats at a quarter of its normal rate, slower than the rate of a person in a coma. Senses disappear. The brain enters a dream state.
Life in the first few hundred feet of the sea is much like life on land, only there's a lot more of it. The ocean occupies 71 percent of the Earth's surface and is home to about 50 percent of its known creatures -- the largest inhabited area found anywhere in the universe so far. The depth of shallow waters, called the photic ("sunlight") zone, varies depending on conditions. In murky waters of bays near the mouths of rivers, it might extend down to only about forty feet or so; in clear, tropical waters, it can reach down to around six hundred feet. Where there's light, there's life. The photic zone is the only place in the ocean where there's enough light to support photosynthesis. Although it makes up only 2 percent of the entire ocean, it houses around 90 percent of its known life. Fish, seals, crustaceans, and more all call the photic zone their home. Sea algae, which makes up 98 percent of the biomass in the ocean and can grow nowhere else but in the photic zone, is essential to all life on land and in the ocean. Seventy percent of the oxygen on Earth comes from ocean algae. Without it, we couldn't breathe.
Having your body pressurized to 36 psi for extended lengths of time can produce mild delirium. At higher pressures, more nitrogen dissolves in the bloodstream, eventually producing the same effect as nitrous oxide, or laughing gas. The more nitrogen in the bloodstream, the more whacked out the aquanauts feel. By the end of a ten-day mission, the whole group is on the equivalent of a Whip-It bender.
Only water could trigger the dive reflexes, and the water had to be cooler than the surrounding air. As it turns out, the tradition of splashing cold water on your face to refresh yourself isn't just an empty ritual; it provokes a physical change within us.
The most incredible transformation, she said, happened at around forty feet down. There, the force of gravity seemed to reverse; the water stopped buoying your body toward the surface and instead started pulling you deeper. This was the "doorway to the deep," where everything changed, and anyone could pass through it -- even me.
She explains that the body responds to extreme breath-holding in three stages. Convulsions are the first-stage response. "You start reacting not from the lack of oxygen, but from the buildup of carbon dioxide," she says. "When that starts, it's just a caution that you've only got a few minutes to go before you really need to breathe." The second-stage response occurs when the spleen releases up to 15 percent more fresh, oxygen-rich blood into the bloodstream. This usually occurs only when the body goes into shock, an extreme state whose symptoms include low blood pressure, rapid heartbeat, and organ shutdown. But it also happens during extreme breath-holding. A freediver anticipates the spleen's delivery of fresh blood, feels it happen, and uses it as a turbo-charge to dive even deeper. The third-stage response is the blackout, which happens when the brain senses that there's not enough oxygen for it to support itself and so shuts off, like a light switch, to conserve energy. Freedivers learn to sense the arrival of convulsions and spleen release, and they know exactly when to head back to the surface so the third-stage blackout won't occur.
Like almost all competitive divers, Trubridge says he dives with his eyes closed. He'll open them for a moment when he reaches the plate at the bottom of the rope, but that's it. By diving blind, he prevents his brain from using up the energy--and oxygen -- it would take to process visual information.
Humans also have magnetite deposits. They're found in the skull, specifically in the ethmoid bone, which separates the nasal cavity from the brain. The location of these deposits in a human head corresponds closely to their position in sharks and other migratory animals -- a relic from the magnetosensitive fish from which humans and sharks both evolved five hundred million years ago. Whether or not modern humans can use the magnetite deposits or some other receptors to attune to the Earth's subtle magnetic field is still not known. But three decades of scientific trials suggest it's possible.
Human magnetoreception is an unconscious, latent sense; we can't feel it turning on or off in the same way that, most of the time, we don't notice that we're breathing. In this, magnetoreception is like the Master Switch; we don't know it exists unless we put ourselves in a situation in which we have to use it.
Dolphins can detect the shape, position, and size of larger objects from up to six miles away. Their echolocation is so powerful and sensitive that it can penetrate over a foot deep into sand; it can even "see" beneath skin. Dolphins can peer into the lungs, stomachs, and brains of the animals around them. With all this information, scientists believe dolphins can create the equivalent of an HD-quality rendering of objects nearby -- not only where these objects are, but how they look from the inside out. In essence, dolphins and other cetaceans have x-ray vision.
Freedivers should never dive alone, and why we must always watch our diving partners for a full half minute or longer after they surface. Freedivers very rarely black out on the seafloor. They black out at the surface, then sink back down and drown.
The first step in saving a blacked-out diver is yelling "Breathe!" in his ears and calling his name. In the blacked-out state, vision and physical sensation disappear, but hearing remains, and it's often heightened. Yelling, Pinon says, activates parts of the brain that have not yet shut down. This jolt can override the body's reflex to close the throat so fresh air can enter the lungs. If yelling doesn't work, we have to remove the diver's mask, tap his face, and start blowing on his eyes. The technique frequently revives blacked-out freedivers; often, they'll come to and begin gasping for air. Now, if tapping, yelling, and blowing all fail to wake the diver, Pinon says, "things get more serious." We need to open the throat and force air into the lungs.
Dairy, he says, can plug the sinuses and make it hard to equalize at depth. Caffeine will raise the heart rate and speed up metabolism, causing the body to suck up more oxygen and shortening dive times.
Of all the disciplines in freediving, static apnea, a timed breath-hold that usually takes place in a pool, is the strangest. It's boring to watch, painful to do, and tedious to train for. And yet there is no other activity that will better prepare a freediver to handle the mental and physical stresses of deep diving.
The partners begin chanting the warm-up breathing pattern -- "Inhale, exhale, hold two-three-four-five-six-seven-eight-nine-ten, hold two, inhale one."
The human body in its natural form -- with little or no clothing -- has the ideal density for freediving; no weights are necessary to aid its descent. However, the thick wetsuits we're all wearing throw off this balance, requiring each of us to wear about twelve pounds of weights in fresh water to compensate for the extra float.
The key to a successful deep dive is making oneself as hydrodynamic as possible. Loose clothing, extended limbs, or oversize masks can create drag, which will slow the descent and decrease depth and "down time" -- freediver lingo for being underwater.
Sinking is relatively easy, especially after the first ten or so feet; ascending is less so, which is why freediving can be so dangerous. As with mountaineering, you need to know your exact halfway point and have at least 60 percent of your energy and oxygen reserves left to make the return trip.
During the ascents, we'll need to exhale all the air we've been holding at about seven feet below the surface. This allows us to immediately inhale much-needed fresh air at the surface without taking time to exhale, and it also helps protect against shallow-water blackouts.
The problem for me, and for most beginners, is equalizing. The optimal rate of descent for a freediver is three feet per second, which requires equalization in sinus cavities (making the ears pop) about once a second, otherwise you'll risk serious injury to the ear. Most freedivers and some jet pilots (who need to equalize quickly during ascents and descents) use the Frenzel method, which traps air inside the closed circuit of the sinus cavities and allows for immediate and thorough releases of pressure. This method is complicated and many people do it wrong, which can cause serious problems at depth. I hire Ted Harty, the team captain for the U.S. freediving team, to lead me through a thirty-minute training session on Skype.
As I pull with my left hand, I reach down and pinch my nose with the right, lift a puff of air from my stomach into my head, then cough a T sound into my closed mouth and seal my throat with my epiglottis. I jackhammer that trapped air from the back of my throat up into my sinuses. It's the first time I've used the Frenzel method at depth. It works seamlessly.
I focus my efforts on pool (underwater laps) and surface training.
I practice static breath-holds while splayed on a yoga mat in my living room. Dry runs are no more tolerable than wet, but they serve a unique purpose: they help me get used to carbon dioxide buildup in my body. Freedivers condition their bodies to tolerate high levels of CO2 using timed breath-hold exercises called static tables. Essentially, it's interval training. Breathe two minutes, take four huge breaths, hold breath for two minutes; breathe one and a half minutes, take four huge breaths, hold for two and a half minutes, and so on. The aim of static tables is to increase breath-holding time while decreasing the rest interval. Within a few weeks, I hit my goal of three-minute breath-holds with only one-minute rests in between.
There was another, seldom-discussed side effect of static training that went beyond increasing CO2 tolerance: it gives you a bone-deep high. This high falls somewhere between the endorphin rush of intense exercise and the dirty, intoxicated feeling you get from drinking bad alcohol in a hurry. A warm spaciness takes over and you feel the electric pulses of your nerve endings firing through your entire body, or you're at least high enough to imagine that something like that is happening. Your mind wanders to happy places.
One of the best surface-training methods is so-called walking apnea, which involves holding your breath and walking over a soft surface (in case you pass out) for extended distances. The idea is that the oxygen your muscles use when you're walking slowly is about the same amount of oxygen muscles use during a freedive. You start by holding your breath while standing still for about thirty seconds until you feel your heart rate decrease, then you walk slowly in a straight line, turn around when you feel you've reached your halfway point, and walk back to your starting place. The distance you travel is about how far you'd be able to hold your breath during a deep dive.
The Manual of Freediving, a 362-page bible of the sport.