I have a feeling that you may have to specity the exact mechanic of the force field. If the air-breathing area is at pressure, we're going to have to talk about toxicity of gasses. If the air-breathing area is at low pressure and the water outside is at high pressure, the most intuitive results would be that they hit the wall like it was a solid concrete wall. The pressure of the water would force you back with all the emphasis that a solid would. For perspective, a water cutter used to cut steel uses half the pressure you'd see at 1km down.

Alright. Your comment really changed my perspective about some stuff. But I feel like commiting to the original idea (at least for now, to see where it goes). The actual point I wanted to make here was: If I could get you from where you are right now and magically place you 1km down in the ocean, what would happen to your body?

And thanks a lot for the insight on the "concrete wall". That REALLY looks cool. I had no idea about the water cutter.

Are your humanoid fish people amphibious? Why do they need air-filled cities, and at much lower pressure level?

This has to do with the plot. But, to sum it up, they keep human prisoners there.

@Magus Worth noting: 1km down is actually believed to be roughly the limit of humans because at those pressures, basically every gas is toxic, even oxygen. Heck, even helium. That would be a very good reason to keep the air volumesa t low pressures.

That's actually a VERY USEFUL piece of information for me, Cort. Thanks a lot.

Checkout mantis shrimp - the 'implosion' of water caused by the speed of their punch is such that it causes light to appear. I imagine something spectacular happening if it was on the scale of a human being! :D.

@CortAmmon Do you do poison? Could some one be put under so much pressure that the very oxygen they breath would kill them? Would it horribly disfigure their face? Could it disfigure a face like this? youtube.com/watch?v=EEyUeCyXl1Q

@Shufflepants Actually, it's quite easy for the oxygen to kill them. Any time the O2 fraction goes above 1.4atm, it is fatal rather quickly. Anything higher than 0.5atm kills eventually. Deep diving technical divers have to use exotic mixes with lower %-O2 than they breath at the surface so that the partial pressure doesn't get to them at depth. Helium toxicity starts to become an issue around 150m, if memory serves, but there's ways around it until roughly 1km. We don't fully understand why helium is toxic, but it does bad things to the nervous system.

You might be interested in Barotrauma. See: en.wikipedia.org/wiki/Barotrauma

From what I've been told... if you're wearing a diving helmet you get put inside it.

@CortAmmon looks like the current 332 m SCUBA dive record was with just trimix (N2, O2, He), and there's also hydrox (H2, O2) which has been shown to work down to 700 m in a simulated dive. Not sure if hydreliox (H2, O2, He) is supposed to be better or maybe just safer than hydrox? Hydrogen narcosis supposedly is "detectable" at around 24 atm of partial-pressure (500 m * 49%)

giant.gfycat.com/HospitableConcreteAtlanticridleyturtle.gif

@Mazura Correct. See the result of the Mythbusters' experiment for (gory) proof.

en.wikipedia.org/wiki/ARA_San_Juan_(S-42)

This comment is meant for @MonicaCellio (although I don't know if she's gonna be notified). I'm keeping the original title of the question, because I want to know the effects of extreme pressure on a human being, not a fish-person.

@Magus got your ping. Can you try to think of another way to make this question title, devoid of context, look less alarming on the Hot Network Questions list on, say, Politics or SO?

Sure. I'll try to figure something out (sorry if my previous comment felt kind of blunt. It was not intented and I just realized now that it might've been).

@MonicaCellio Please tell me if the new title is okay :)

Better -- thanks for understanding!