Welcome to Science of the Scare! Every month I will dissect a Big Science Question from a horror movie and talk through it in (mostly) easy-to-digest terms.

Science and horror have a wild, entangled history and have left us with loads of questions to ponder. Deep, important questions like: just how many ways could we have a zombie pandemic? Is genetic engineering always a slippery slope to monstrosity? This month’s Big Science Question:

Could breathing compressed air really make someone experience an extended hallucination sequence?

This question comes to us from real life, as well as Johannes Roberts’s 2017 underwater horror 47 Meters Down. In this compact 85-minute shark-infested feature, two sisters, adventurous Kate (Claire Holt) and careful Lisa (Mandy Moore) decide to board a janky boat to experience an underwater shark feeding within a diving cage. When the cable holding their cage frays and snaps leaving the sisters stranded — you guessed it — 47 meters down on the ocean floor, they have to try to find a way back to surface before air runs out.


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Kate is a certified diver, but Lisa is new and clueless to breathing underwater, so she gets constant warnings of all of the risks involved: stay calm and conserve your air because freaking out will just deplete your tanks; don’t zoom back to surface or you’ll get the bends; watch out for signs of nitrogen narcosis. Note that the common threat for all of these isn’t the massive sharks swimming around. That’s because the biggest risks to diving have to do with gases under pressure.



The bends, or decompression sickness, is probably most well known issue that can strike divers (and there are a lot of issues that can strike). The whole point of breathing is to take in gases, including good ol’ oxygen, that are dissolved into our blood and transported to tissues throughout the entire body. Thanks to technology, we have no problem breathing compressed gases underwater and can go from low to high pressures with relative ease. It’s going from a high pressure environment to a low pressure environment that the bends become a problem. If divers ascend too rapidly, the gases, namely nitrogen, dissolved in the blood expand, come out of solution and form tiny bubbles in the tissues and blood. These bubbles can cause all sorts of trouble, including blocking blood circulation from parts of the body ( 47 Meters Down preferred to focus specifically on bubbles forming in brain tissues, which you don’t need me to tell you would be capital B Bad). This is why, when there’s an attempt to haul the fallen cage back up with a spare cable, it’s brought up very slowly, pausing every five or ten metres or so for five minutes so that Kate and Lisa‘s bodies can adjust to the pressure decreases as they ascend. Risk for the bends increases the deeper the dive and the longer you’re down and breathing from a tank, and Kate and Lisa definitely qualify as high-risk on both counts. 

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The idea of gases bubbling within your blood is horrifying to imagine, but there’s something somehow scarier about narcosis. In 47 Meters Down we spend a long chunk of time wrapped inside Lisa‘s narcosis hallucination. In her mind she frees her trapped leg from where it’s pinned beneath the doubly fallen diving cage, she finds her injured sister and heroically brings them both to surface while fending off sharks with underwater flares. Like a dream, it’s hard to tell just how much time has actually passed, but when we jump back to Lisa, who never freed herself, and definitely never jabs a shark in the eye, she seems so happy and unaware that she never left the ocean floor.


“[The threat] isn’t the massive sharks swimming around. That’s because the biggest risks to diving have to do with gases under pressure.”


Any gas can cause narcosis, but with diving, nitrogen tends to be the culprit. When Kate and Lisa are dropped second air tanks, they’re warned to watch out for signs of narcosis. They’ve been breathing compressed air for a while with no way to safely get rid of the dissolved nitrogen building up in their blood. Switching to a second tank only serves to up that concentration and create toxic effects. The risk only gets worse the deeper the diver is. Generally speaking, the cutoff for recreational diving is usually around the 40-meter mark because of the risks that increase the deeper you dive, so while not technically a deep dive (which would be categorized as around 60 meters or deeper), Lisa and Kate are well below where recreational divers should safely go, especially a diver as inexperienced as Lisa. 

Just like how alcohol inebriation can affect people differently, different divers might experience nitrogen toxicity differently. In mild cases, it could be kind of like the experience of being drunk (except underwater) or you might think that fish are talking to you, which doesn’t sound too bad, but if you’re distracted while diving you could be putting your or your dive buddy at risk; in more severe cases, you might experience convulsions. Lisa‘s hallucinations prevent her from focusing on keeping herself alive as she waits for rescuers. I once worked on a project that interviewed a freediver — yep, you can get narcosis even if the only air you’re swimming with is your own two lungfuls — who experienced narcosis as euphoria. She described it as being overcome with such peace and happiness being underwater that she didn’t want to go back up to surface. She might have died if she hadn’t had a diving buddy because she was taken by the rapture of narcosis.  



Is a diver’s personality a risk factor in this 47 Meters Down scenario? Does being the type of person who seeks out the thrill of jumping into shark-infested waters or sky-diving make you more or less likely to survive? The movie traps us underwater with two very different personalities to explore this idea to a degree. Some research has shown that people who are high sensation seekers, that is people who are more likely to try out extreme experiences like freediving or bungee jumping or BASE jumping tend to get less of an adrenaline rush from the experience — that is, they feel the thrill, but it’s not engage the so-called fight-or-flight response that a more anxious person might experience. It’s not uncommon for thrill seekers to feel a sense of calm and get dopaminergic effects (aka good vibes from the brain’s reward system). These people also tend to be well-versed in the risks of the activity and how to mitigate them, and keeping a calm head and clear mind through the experience can only enhance their chance of survival in an unsafe scenario. Underwater, panicking will only deplete your available air and leave you unable to take action and vulnerable to everything that might kill you.


“Does being the type of person who seeks out the thrill of jumping into shark-infested waters or sky-diving make you more or less likely to survive?”


Do you think of yourself as more of a thrill-seeker like Kate or a more reticent chill-seeker like Lisa? If you’re where you fall on that spectrum, you can actually take a survey and Dr. Ken Carter, author of Buzz! Inside the Minds of Thrill-Seekers, Daredevils and Adrenaline Junkies and clinical psychologist interested in understanding how the minds of so-called “adrenaline junkies” work, will email you the result of where you fall. I am on the weenie end of the spectrum with a score of 7, and prefer to seek my thrills in what I think are safe ways: watching horror, exploring cities, and trying all sorts of strange (to me) food. If it’s your thing, great! But you won’t catch me cage diving anytime soon. 

Have a Big Science Question from horror that you’d like to see answered? Let us know over on Twitter, Reddit, and in the Horror Movie Fiend Club on Facebook!