Going Aquatic Part 5: Underwater Weirdness

I know I promised some movie analysis to help with your games, but the rewatches, note taking, and all that are more time consuming than anticipated. Not to worry though! Because this is the world building article you didn’t know you needed! Time to talk about how going underwater makes things freaky so you can ramp up the tension at the table when your players take the plunge.

WARNING: this is a broad overview of some of the science behind diving and the physiological effects it has. It’s not comprehensive, and there’s whole books on the topic. Diving without proper training is dangerous in the extreme. This article is just a collection of underwater effects and realities and how they might be applied to a tabletop RPG.

Eternal Blue

This is more than just a pretty term. As you descend into the water, colours are slowly filtered out until only blue remains, then darkness. Around 25m or so, Reds, Oranges, and Yellows get “washy” and are less distinct, even “brownish” by 50m. Blue to Violet got deeper, with blue making it all the way to 200m. And this makes things look WEIRD. It’s one of the reasons why activating a white light at depth can reveal shocking colours and things you missed otherwise.

So how can you use this? It makes spotting things without a light more difficult and allows things to hide or be hidden better. It’s also a way to conceal messages or communications. In terms of atmosphere, it’s a great tool to let the player’s know how strange the world their characters are descending into is.

Sound Problems

Believe it or not, sound moves through water faster than air. And this is a problem to our air adapted ears. Basically, if you’re not looking directly at the source of a sound? You have to actively search for it. It’s why the drill for “I’m diving near the surface and hear a boat” is to dive deeper. Then there’s ambient sound. This can be minor (blue water diving away from anything) to serious (parrotfish crunching coral on a reef) to impossible (heavy machinery operations).

In game terms, this is another sensory based effect that adds challenge to being underwater. It’s distracting, it’s a problem. It’s why having sonar or echolocation is so important to detect things. Another use is to weaponize it, literally. In games where’s sonic attacks, they’d likely be more effective underwater!

Crushing Depth

This is always popular in science fiction and kind of reality; so, let’s look at it. The first thing to understand is that it’s not just “pressure” that’s the problem in this case, it’s that there’s different pressures present in the same place. Nature doesn’t abhor a vacuum, but it does HATE a scenario where there’s different levels of pressure. This works a few ways.

The one too many people are familiar with now is the idea of the deep sea crushing a submarine or submersible. This happens in pressurized conditions where the inside of a vehicle has a lower atmospheric pressure than the water outside. A classic diving lesson about this is to take an air-filled plastic milk carton down 20m and watch it crush. This is why submarines, submersibles, and atmospheric diving suits have test depth (the normal operating depth), never exceed depth (the last depth where hull integrity is reliable), and crush depth (generally where the sub will implode). This scenario means that the vessel implodes on the crew, crushing inwards in a fraction of a second.

Differential pressure is the pressure situation divers worry about. Why? Because broadly speaking, the human body is pretty crush proof at depth. Yes, we use more breathing mixture, and some freaky stuff happens with nitrogen bubbles in our many tissues, but we’re kind of okay as long as our gear keeps working (never push gear past its max depth!). But then along comes differential pressure, or “delta-p”. If the one-way valve on a dive hat fails, then the ocean will try to stuff a body into the hose gear connecting them to the surface. Why? Because the surface is lower pressure than depth and nature hates that it’s connected to a higher pressure place. Then there’s the “hole in a pipe” scenario, best exemplified by the following video. CONTENT WARNING: This is an actual commercial diving training video and features CGI recreated delta-p incidents and actual footage of a crab dying from delta-p. 

So how to use this in game? For crush depths, it’s a limit of exploit. The characters can’t safely go past it without rapidly endangering themselves and their gear. It’s a way to drive tension and because it’s a game and not real life, some risk taking because engineers generally design things to be a bit tougher than their rating and sometimes you just get lucky. For delta-p, it’s a good hazard to throw into a battle area as an environmental hazard, or to use as a trap or even a plot point.

Unfriendly Bubbles

Remember those nitrogen bubbles I mentioned? Yeah, so, pressure makes gases we normally breath get risky under pressure. Our surface 21% O2 79% Nitrogen air is, in fact, fatal at depth. Around 66m down you gain the risk of oxygen toxicity. But of course, we’re human, so why pay attention to that? For deep dives, divers often switch to TriMix, a combination that replaces some of the oxygen and nitrogen with helium. For super deep dives, to 300m, they may switch to HeliOx, a mix of helium and oxygen where the O2 might get as low as 6%. Deeper than that and it gets stranger, with hydrogen replacing the helium to make HydrOx. The deepest “dive” ever was a trip to 701m down in a hyperbaric chamber and used a variety of breathing mixtures to take the diver down and back. Ironically, in shallower dives, increasing the % of O2 can make things safer, and mixes of NitrOx between 28% O2 and 40% O2 are popular for both recreational and commercial divers.

The back bit is the real issue though. Because returning to the surface from any dive needs a safety stop to aid the body in decompressing. The deeper you go and the stranger your dive’s breathing mixture(s) get, the longer this might take. We’re talking hours to weeks. The latter is done in a decompression chamber though as opposed to in water.

Now, why do all this mixing and stopping? It’s to avoid things like Decompression Illness (DCI, “The Bends”, nitrogen bubbles in your joints): nitrogen narcosis (when your brain gets drunk without booze at literally depth without rhyme or reason); or High Pressure Nervous Syndrome (HPNS, “the helium tremors” with a side of visual disturbances and more) that can happen when you breath too much helium past 150m.

In game terms, these influence two things. One is the technical flavour of the game; it’s something that can affect maximum depths, provide GMs with consequences for reckless actions or desperate ones… It’s a bit in the weeds but if that’s how your table is rolling, it’s AMAZING.

Making it Cinematic

Real world diving is some risky stuff, but for games, it’s cinematic. In the real world, a rapid ascent from 200m down is a serious event that’s likely going to result in medical intervention. In a game, it might just be some minor inconvenience. So how can you make all this work for your game?

The first thing is that right away, in session zero, you need to figure out how technical the players want to make things, then balance that with fun and game flow. Some things, like colour shifts, lights, and sound are easy to integrate into any game. They’re modifiers to rolls and descriptive text about how things are. Delta-P and Crush Depth are more environmental additions and of them, the latte has the most utility. Crush depth in a highly technical game is a hard limit, in a highly cinematic one, it’s a suggestion that leads to exciting runs through collapsing areas with water spraying and the sound of metal groaning. Breathing mixture and decompression is either a note in a highly cinematic game and maybe some minor damage for taking things past the limit, or it’s a set of serious limiters and boundaries in a highly technical game.

Final Thoughts

The technical aspects of the underwater world are many and varied. They’re also not too well known outside of the diving and underwater world. And as a diver myself, I’m uniquely positioned to help bring this info to the creatives wonder about it. So I hope this short guide gives you something to think about for your next aquatic game or setting!

Read the whole series! [Part 1][Part 2][Part 3][Part 4][Part 6][Part 7][Part 8]