Forget The Sharks: How '47 Meters Down' Fails Dive Science

This is a guest post by Jake Buehler, who just so happens to be an AAUS certified scientific diver as well as a science writer based in the Seattle area. He blogs over at Sh*t You Didn’t Know About Biology, which is full of his “unrepentantly celebratory insights into life on Earth’s under-appreciated, under-acknowledged, and utterly amazing stories.”

Summer is finally here in the Northern Hemisphere. The days are long, the weather is warm, and the water is inviting. It’s also time for our annual lesson from popular culture that this refreshing invitation is a lie, and that the only thing the sea offers us is electric, blinding terror. Yes, summer inevitably means the advent of a new crop of shark-based survival horror flicks.

This summer, much like the last with “The Shallows”, movie-going audiences will be treated to another shark-centric screamfest: “47 Meters Down.” The British-American film — starring Mandy Moore and Claire Holt — opened in U.S. theaters last weekend.

Recent, somewhat ubiquitous trailers for the film outline its terrifying premise: While vacationing in Mexico, a pair of sisters goes cage-diving with great white sharks, only to have the winch suspending their protective cage fail, sending them plummeting 47 meters down to the ocean floor, from where they must escape to the surface before the swarm of sharks — or their dwindling air supply — does them in.

It is no doubt that just like with “The Shallows”, we will again be reminded that the persistent blood lust of horror film sharks is altogether different from what science tells us about the behavior of their real-life animal counterparts. But the film and overall premise of “47 Meters Down” commit a litany of science inaccuracy sins completely unrelated to sharks.

Frankly, the movie fails spectacularly when it comes to portraying the biology and physics at play during SCUBA diving (which is kind of amazing, actually, considering how much of the film’s plot is directly rooted in the consequences of being underwater). Being a trained AAUS scientific diver, dive science is an area I know a little about, so I made the commitment to sit through “47 Meters Down” so you wouldn’t have to, all to separate the reality of how diving works from … well, whatever it is that the movie plopped out.

The backdrop of the haline horrorshow plays out like this: sisters Lisa (Mandy Moore) and Kate (Claire Holt) are on vacation in Mexico. Lisa reveals that the idea for the trip was largely spurred by her recent breakup with her boyfriend, who left her because he got “bored” with her and their relationship. What better than a spontaneous trip down to Mexico to prove him wrong?

Kate attempts to brighten her sister’s spirits by taking her out drinking and dancing until the wee hours of the morning. In doing so, the sisters meet some local men who tell them about cage-diving with sharks, convincing them to join in the next day. Lisa is terrified of the idea — both beforehand and the next day when they actually make it out to the boat — but Kate reminds her that posting cage-diving photos on social media is a surefire way to cultivate jealousy in her ex.

The cage-diving operation is beyond sketchy (something explained away by one of the local men as “it’s Mexico *shrug*”). The captain of the boat and this whole cage-diving outfit is Taylor, portrayed by a curt, sun-withered Matthew Modine — a man heading an alarmingly rusted and worn down vessel, and wholly unconcerned with whether or not the sisters actually know how to dive in the first place (Kate has some dive experience, but Lisa has absolutely zero, despite telling him otherwise).

As the sisters gear up and prepare to descend in the cage, great white sharks start to circle the boat, exciting basically everyone other than super-skeptical Lisa, burdened with the heavy hand of foreshadowing. Some of them are huge, as much as 28 feet in length, according to Captain Taylor (which is waaaaay bigger than any recorded great white shark in real life).

Eventually, after a very short safety briefing from Captain Taylor, Lisa and Kate sink just below the surface in the cage, marveling at the shimmering schools of scombrid fishes, and the giant, toothy stars of the event. The two sisters converse with each other (and Taylor up top) using a comm system embedded in what appear to be the most incredible, top of the line full-face masks ever (more on that later), sharing in the excitement.

Soon, Lisa decides that she’s had enough shark-induced adrenaline for one day, and that they need to be pulled back up. Of course, the Murphy’s Law of horror flicks like these kicks in immediately, and the winch suspending the cage busts, sending them rocketing down to the ocean bottom. It is at this moment that the movie starts sharing “alternative facts” about what happens at depth, which I have broken up below.

Once the cage begins to plummet to the seafloor, Lisa and Kate begin to experience increasing pressure. As more and more seawater stacks up above them as they sink, the pressure exerted on their bodies by all this water goes up. Every 10 meters or so of seawater produces the same amount of pressure as the entirety of the Earth’s atmosphere at sea level. So, at 10 meters down, a person experiences 2 atmospheres of pressure. 20 meters down, it’s 3 atmospheres, and so on. All that squeezing doesn’t do a whole lot to your solid, fleshy parts — but it does a lot to air spaces. Gas is compressible, and as the pressure mounts, the air-filled sinus cavities, interior parts of the ears, inside of the mask, and lungs are all impacted.

In SCUBA diving, one of the first things you learn is how to combat this pressure upon descent using equalization techniques. This is particularly important for the ears, because if you reach a mere several meters down and fail to equalize any of that pressure pushing in, you can be dealing with a serious amount of pain, ruptured eardrums, and even irreversible hearing loss. The only real way to avoid discomfort is to descend slowly and equalize as you go, and by slowly, I mean half a meter at a time.

Lisa and Kate do not descend slowly. The cage they are trapped inside of hurtles towards the bottom at a rate that made me wince in sympathy for their poor, doomed air spaces. Once on the bottom, Lisa is shown to have a minor bloody nose, which is the extent to which either of them have any adverse effects from the pressure (also known as “barotrauma”). Increased pressure in the sinuses can definitely cause blood vessels to pop, and this can even happen under normal descent conditions.

But because the two sisters are using full face masks and not “conventional” half masks, barotrauma in the sinuses and within the air space of the mask is essentially impossible (the constant flow of air into the full face mask keeps the interior pressure regulated with every breath).

Had Lisa and Kate been wearing conventional masks, it’s likely their free fall to the bottom would have resulted in a catastrophic, excruciating degree of “mask squeeze”, where the outside pressure of the water crams your mask into your face, causing nasty, extensive bruising and your eyes to turn the color of merlot, making you look like Qyburn just brought you back to life.

Rather than mask-associated barotrauma, it is far more likely that Lisa and Kate would have blasted out their ear drums after rocketing down to more than five atmospheres of pressure, flooding their ear canals, hitting them with a bout of vertigo and persistent deafness. But, all they have to show for their extremely fast descent is a single barotrauma-induced leaky blood vessel in precisely the wrong location.

As they fall to the bottom, there’s a dramatic shot of the display face of a wrist-mounted dive computer cracking, illustrating the extreme depths and pressure Lisa and Kate are plunging towards. Wrist-mounted dive computers are basically high-tech watches designed to relay information to the diver about the environment (current depth, temperature), how long they’ve been below the surface, and other important info. These devices are flat-out rugged, often outfitted with tough displays made out of scratch-rebuffing mineral or sapphire glass, and rated to accurately register depths of 100 meters or more (like the popular, no-frills Geo 2.0 computer put out by Oceanic).

These depth ratings? Yeah, they aren’t for the structural integrity of the display or the body of the computer; they’re for the pressure sensor. A computer that starts to inaccurately register depth at 100 meters might eventually fail to be waterproof somewhere below that point, but acrylic or specially tempered glass crushing like a Jolly Rancher under a boot heel? If this could happen to a dive computer at all, it wouldn’t happen at 47 meters, and if Lisa and Kate were actually exposed to a hyperbaric environment capable of such a feat, an obliterated inner ear would be the least of their worries.

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Once Lisa and Kate are down at 47 meters, the dire nature of their situation becomes clear. Strapped to their backs are air tanks, filled with compressed air that they are currently breathing — a tragically finite resource. These tanks appear to be a variety pretty standard in the SCUBA world: “aluminum 80s”, named for their composition and the fact that they are rated to hold 80 cubic feet of gas.

This registers on a pressure gauge as about 200 bar when full (or for imperial folks, a little over 2900 psi). The pressure in the tank, expressed as a number in bar or psi, is a ticking clock on any dive. It is the most obvious limiting factor in a dive, and everything you do underwater fits into a plan to manage your air supply, so that you come up to the surface alive.

Being stuck in a rusty cage, surrounded by inexplicably murderous wildlife provides a major wrench in that plan.

The film is quite aware that the more time the characters spend underwater, the lower their air reserves get. But what “47 Meters Down” completely and spectacularly ignores is that, with SCUBA, depth is incredibly relevant to the rate of air consumption. With increasing depth and increasing ambient pressure, the air you breathe on SCUBA becomes more dense. But because the volume of your lungs doesn’t change, you end up using a greater volume of your overall air supply with every breath, the deeper you go.

With every additional atmosphere of pressure, the air you breathe is compressed to half the original volume. So, at 2 atmospheres (about 10 meters below the surface), your lungs take in air that is twice as dense as it would be at the surface, meaning you will go through your air twice as fast. At 47 meters down, a person is experiencing 5.67 atmospheres, which means they blaze through their air at nearly six times the rate they would at the surface.

This relationship between depth and relative air consumption rate is not possible to get around, and the fact that Lisa and Kate are so damn deep makes just about everything they do in the film underwater either physically impossible or fatal in the real world. To really illustrate why this is the case, we need to get an idea of how long a diver can be expected to last on a tank of air at 47 meters depth. The base level of air consumption (often shared as a surface air consumption (SAC) rate, which is the cubic feet used per minute at rest at the surface) can vary quite a bit between people, and can be influenced by physical attributes like fitness (less conditioned divers might blow through air faster) or body size (bigger folks tend to need more air, unsurprisingly), or dive experience, as a familiarity and comfort with diving can increase air use efficiency.

Some people, for unclear reasons, just manage to use very little or large amounts of air (I’m one of the latter; I can drain a tank like no one’s business). Temporary factors like stressful or tiring dive conditions can decrease efficiency on a given dive. A pretty normal SAC for a comfortable, experienced diver can be ballparked at 0.5 cubic feet per minute, meaning that a full aluminum 80 tank could theoretically last 160 minutes at the surface. At 47 meters, it would take about 28 minutes to drain the tank to empty … not considering the time breathing on the tank before and during descent, or leaving enough air to get back to the surface.

Again, this is a calm, resting, experienced hypothetical SCUBA diver. Lisa and Kate, to put it lightly, are none of these things. They spend huge chunks of time panicking, screaming, and swimming as fast as possible away from ravenous great white sharks (an action, I might add, that is made all the more difficult due to neither of them even wearing fins).

At one point, Kate is engaging in multiple feats of strength in an attempt to free her sister from the cage, which is a level of exertion that cannot be kind to her overall air reserves. Not long before this, Kate advises her terrified and hyperventilating novice diver sister to breathe slowly and regularly to conserve air — not that any benefit from this is realized when the two of them are executing a continual Cross Fit workout at depth.

We can give a very conservative estimate that the insanely strenuous and stressful conditions would give Lisa and Kate at least the equivalent of an SAC of 1.0 cf per minute (for some reference, I’m a big dude who has had mildly stressful dives while relatively inexperienced that came out to an SAC of about 0.9). At this rate, Lisa and Kate would empty their bottles in less than 15 minutes … not even considering all the time they spent in the cage at the surface.

In the real world, Lisa and Kate would never have the opportunity to flee from the horde of sharks. They would run out of air and asphyxiate long before having the opportunity to escape from the cage. The movie would run about as long as the sequence of previews that came before it.

“47 Meters Down” doesn’t help this dramatic miscalculation when it brings up their remaining air pressure multiple times throughout the movie. In one moment, Kate estimates that 80 bar will last 20 minutes at their 47 meter depth. She is wrong. Under ideal conditions (where SAC is 0.5 cf per minute), that might get her 11 minutes. Realistically, it’s probably half that amount of time.

Of course, what follows is the bulk of the movie, arguably longer (even accounting for some attenuation of the duration of events) than her own exaggerated number, and definitely not a handful of minutes. Long after this point, Lisa gains access to a second air tank, but even under the most optimistic scenario, there is just no way she’d last long enough on the first one to enjoy it.

Read more: Why Do Sharks Attack Humans?

A third conflicting factor severely messing up Lisa and Kate’s dive — you know, outside of the sharks and the rapidly draining air supply — is the risk that comes from coming up to the surface too quickly. The culprit is nitrogen, which makes up nearly three-quarters of the composition of air, and is normally inertly inhaled and exhaled as our physiology focuses on the “good stuff” (oxygen).

At depth, all that compressed nitrogen gas dissolves and accumulates in the blood and tissues. All of that is all well and good, but when the dive is over and you head to the surface, the pressure drops, and all those dissolved nitrogen bubbles are free to expand in a process not all that different from the carbonation fizzing in a freshly-opened soda bottle.

If you come up at a pace slow enough to let your tissues off-gas their nitrogen load, everything is golden. But if you come up too fast, you can get nitrogen bubbles forming in your tissues as they decompress, causing “decompression illness” or “the bends.” DCI can range from transitory discomfort or pain, to a life-endangering emergency, and it’s often hard to tell which it’s going to be until the symptoms emerge.

If decompression is handled poorly enough, a diver can have nitrogen bubbles forming and lodging in arteries. This “arterial gas embolism” can be acutely deadly. It’s not just nitrogen bubbles that can be a problem for ascending divers: as the pressure drops, the air being pushed in and out of the lungs swells in volume. Moving upwards slowly while breathing avoids any issues, but shooting to the surface or holding your breath while swimming upwards can cause the lungs to over-expand and burst, sending gas into the bloodstream, causing an arterial gas embolism on its own.

For these reasons, safe ascent is drilled into trained divers’ heads, and a “decompression stop” — where the diver finishes a dive by hanging out a few meters down for a couple minutes to give the nitrogen more time to fizzle on out of the tissues — is very commonly incorporated into even relaxed, shallow dives as a safety precaution.

DCI/the bends is bad. It is worth avoiding at every turn. At best, it is painful and debilitating, and at its worst, it can kill you. What it isn’t, though, is an automatic death sentence, which is what “47 Meters Down” seems convinced of.

Shortly after Kate escapes the cage, she swims towards the surface a bit (up to 40 meters) so she can get in range of the boat, allowing her to use her mask communications system to notify Captain Taylor that they are still alive down there. Kate tells Taylor that she has 55 bar left (that’s only one-quarter of a tank!).

Taylor then advises Kate and her sister stay down in the cage at 47 meters and wait until they can pull the whole thing up somehow, because if they go to the surface while fleeing the sharks, they will get the bends and they will DIE!

This is a ludicrous flipping of priorities. Yes, decompression illness can be dangerous, even fatal in some cases. But you know what’s 100% fatal? Not having any air to breathe. Taylor advising Kate and Lisa to sit tight at 47 meters and race through their air rather than take a risky ascent is like your doctor telling you to stop drinking any and all water because there might be lead in it. Yes, technically lead poisoning is not ideal, but perishing from dehydration is quite a bit worse.

The ultimatum given in “47 Meters Down” for avoiding the bends (and a guaranteed, awful death, apparently) is that Lisa and Kate need to do a decompression stop for 5 minutes at 20 meters down. If they do that, Captain Taylor claims over the comm system, then they are in the clear. If they don’t do this, then “nitrogen bubbles will form in your brain and you will be dead.”

As a precaution, it sounds nice, but after spending something like 45 minutes of film time (somehow) at 47 meters, a single safety stop at twenty meters isn’t likely to reduce the risk of decompression illness much at all. “Deep”, 120 or 130 foot dives using normal air mixtures that last for half that amount of time would need multiple stops at shallower depths to help facilitate decompression of gases.

Captain Taylor’s obsession with decompression safety also immediately, and hilariously, falls apart when, during a scene where the two sisters are on their way to the surface and finishing their five minute, 20 meter decompression stop, he tells them to swim upward as fast as they can to the surface as soon as the stop is complete! The closer you get to the surface, the more dangerous rapid ascent becomes, because at shallower depths is where the bulk of the compression and expansion of the volume of gases occurs. Suddenly jumping up from 80 to 70 feet down isn’t nearly as dangerous as sprinting up to the surface from 10 feet down. Darting to the surface after an insufficient decompression stop is actually the worst possible way to avoid getting the bends. So after an entire film of fixation on avoiding the bends, Captain Taylor tells Lisa and Kate to do something almost guaranteed to cause decompression issues.

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When Captain Taylor floats out the idea of sending down extra tanks to the two sisters, he mentions that using a second tank will increase the risk of “nitrogen narcosis”, and that they would need to be careful and evaluate each other’s behavior for concerning irregularities. Nitrogen narcosis — also called the “rapture of the deep” and the “Martini effect” — is a real phenomenon. It’s an effect of diving on a normal air mixture that typically doesn’t become noticeable until at least 30 meters down.

The mechanism of the narcosis isn’t entirely understood, but it is assuredly a consequence of high partial pressures of dissolved inert gases (nitrogen and noble gases like xenon and argon) in the body of the diver. Narcosis has been likened to alcohol intoxication, or being under the effects of an anti-anxiety drug, and can range from a slight dulling of senses and judgement, to severe confusion in some people. Whatever narcosis is, it is not what is portrayed in “47 Meters Down” at the end of the movie.

Nitrogen narcosis is depicted as a long, complex, “waking dream” style hallucination experienced by Lisa while she waits in the cage for help. Viewers discover that a highly-involved escape and rescue sequence is entirely imagined by a stationary Lisa, unbothered and lost within an episode approaching psilocybin mushroom trip levels of severity and duration.

While nitrogen narcosis hallucinations can occur at the kinds of depths Lisa and Kate experience in the movie, sufferers aren’t transported to fully-formed, alternate realities. For most divers that experience narcosis, the effects are more akin to mild to moderate cognitive impairment, not the impact of powerful hallucinogens. Personally, I’ve experienced nitrogen narcosis on several occasions when diving at around 30 meters; it manifested mostly as difficulty keeping track of time (which can be dangerous while on a deeper dive), forgetfulness and slight disorientation, and problems effectively communicating with my diving buddy.

Narcosis is strongly associated with depth. The effects dissipate very quickly if a diver moves towards the surface. Captain Taylor’s claim that nitrogen narcosis becomes a risk with using a second tank isn’t quite correct; if narcosis was going to be an issue at 47 meters (which it likely would be), it would have been impacting Lisa and Kate long before that point. A second tank leading to more gas saturation would have far less impact than, say, descending five or six meters.

Of course, it should go without saying that the depiction of shark behavior is all wrong, too. Just as with “The Shallows,” there’s no way great whites would behave like this, even if they were offered up a couple of completely clueless, tasty humans. But that doesn’t even matter in the grand scheme of things; if this movie were true to life, Mandy would never have gotten the chance to worry about the sharks.

“47 Meters Down” exists in a world where much of the physics and biology of diving don’t apply, from how air consumption works at depth, to the effects of unmitigated pressure on the inner ear. The survival of the two main characters to the point of dealing with the threat of the great white shark swarm at all is only possible due to the suspension of essentially the entirety of dive science. Perhaps, if you consider it a science fiction movie rather than a horror flick, you can suspend disbelief long enough to make it through the film without groaning audibly at the sheer impossibility of it all.

Then again, based on the reviews, it’s probably not worth trying.

Read more: Almost 20 Million Years Ago, Sharks Nearly Went Extinct