Yeah, like in a submarine..

but I'm not sure if a submarine is pressurized, like a commercial jet, although they're at the opposite sides of the pressure spectrum.. I would assume a sub would need some type pressure to keep from being crushed at depth? But I dunno.. Maybe Iowa would know more about that..

Where you get into trouble diving is when you breath air at depth that is pressurized.. For example, if you take a full balloon and take it to the bottom of a pool, the balloon will get smaller due to the water pressure compressing the air in the balloon.. but if you inflate a balloon at depth, until its full, and then you come to the surface, the balloon will overexpand and burst.. that's how you blow up your lungs by surfacing too fast on a dive or because you were holding your breath..

That's one problem..

The other is that air we breathe is 79% oxygen, 21% nitrogen.. For some reason, when you breathe, your body off-gasses oxygen at a much faster rate than the nitrogen.. so, nitrogen particles/bubbles take longer to get out of your blood under pressure, or at depth.. that's why if you shoot the surface on a dive, the oxygen will off gas out of your body, but the nitrogen won't have time to and you get the "bends", which is nitrogen bubbles stuck in a joint... the bubbles get larger as you surface and they get stuck in an artery or vein because they're too big to pass.. and then you're a very unhappy camper.. or diver, in this case..

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