In short: most of the body can handle any pressure, but if we're breathing air the maximum safe depth is about 60 meters (190 feet), the feasible maximum with current technical capabilities (and special breathing gases) is about 500 meters (but, for safety reasons, those depths have only been simulated). Assuming we can crack a condition called High Pressure Nervous Syndrome (HPNS) that we don't know much about, we might theoretically go down to about 2-4 km (200-400 atmospheres) at which point even helium will probably kill us.



Most of the body handles pressure well, because we're mostly water. We need to equalize air spaces in our sinuses and ears when we dive, but (as most divers experience) this is most noticeable at the surface, where the pressure change (as a percentage) per meter is greatest. If we equalize (as one must do early and often, as I learned painfully on my first dive) we can survive high pressures just fine for the most part. Sudden changes in depth/pressure can be painful or dangerous (especially near the surface) but we adapt to changes in pressure pretty easily, given enough time. There is one factor that makes for most of the difficulty: we may not be aware of it, but we're breathing substantially more gas, with each breath, than we woud on the surface. Up here at 1 atmosphere of pressure, nitrogen is pretty damn boring: N2 is an almost-noble gas with a triple bond: not very reactive, doesn't do much despite being 78 percent of our air. If you're under 30 meters of water, and at quadruple the atmospheric pressure, nitrogen starts to make you high, because you're breathing about 3.12 atmospheres of it. (The specific mechanics of gas narcosis are not well understood.)



For SCUBA on air, recreational diving sets a limit of 40 meters (130 feet) because of the risk of nitrogen narcosis, which is similar to nitrous oxide (laughing gas) intoxication at low levels (30-50 meters) but can cause death at high levels (100 meters). That said, most divers can survive being narked if they're trained. You won't be much for original thought, but you can keep yourself alive and ascend to a depth at which narcosis isn't an issue. Contrary to the image of the diver as a rule-breaker, most intelligent divers follow the rules to the letter: diving is quite safe if you follow the rules, and incredibly dangerous the moment you break them; with narcosis, if you haven't set safe behavior patterns, you could be at risk.



At 60 meters (190 feet) oxygen becomes toxic. This is because you're at 7 times the pressure on the surface, so you're breathing "147 percent oxygen", or almost 1.5 times as much oxygen as you'd take in if you breathed pure oxygen (100% oxygen is, contrary to reputation, safe for short-term consumption at atmospheric pressure; but at a partial pressure of 1.4-1.6 atmospheres, it's a different beast). Narcosis impairs judgment, but oxygen toxicity can cause an epilepsy-like seizure, which is often game-over if it happens at depth. (If this happens and someone is not there to rescue you right away, you will lose consciousness, spit out your regulator, and drown.) This is why people doing the very deep technical dives use multiple gases; at high depth, you will need to breathe gases that would be dangerously hypoxic at the surface.



Nitrogen and oxygen are both too dangerous to breathe at 7+ atmospheres of pressure, so helium is often blended in with the breathing gas. With helium in the mix, divers can theoretically go down to about 300 meters (1000 feet). That's a dicey proposition, because even with exotic gases, you have to decompress for hours after a dive at that depth. Without a lot of special training, diving even to 100 meters is a bad idea; and on air, fatal nitrogen narcosis and oxygen toxicity are severe risks. Plenty of people die trying to reach the bottoms of various "blue holes" around the world, undaunted by the risks that increase exponentially with depth.



In a controlled environment (if something goes wrong at 300 meters, nothing can be done for you; in a chamber, there are more options) people seem able to survive depths in the 500m range and, of course, with atmospheric diving suits, people can go deeper, but that's effectively "cheating" because it's a personal submarine: you're not exposed to the pressure.



It's unclear that there's any gas out there that we can breathe at, say, 100 times the atmospheric pressure. You need oxygen at 0.16 to 1.2 partial pressures (preferably, close to 0.21) and the rest is negotiable, but nitrogen and the heavier noble gases (neon to krypton) become narcotic, and pretty much everything that's not a noble gas (or hydrogen) becomes outright toxic. Helium and hydrogen seem to be the best bets, and even using those gases, the limit seems to be in the 30-50 atmosphere range (300-500 meters). At that point, High Pressure Nervous Syndrome (HPNS) becomes debilitating. HPNS is helium-related but not seem to be a gas narcosis-- in fact, helium seems to be the least narcotic gas-- and there's been some work to try to treat it, mostly pharmacological.



If we could somehow crack HPNS, the next barrier would probably be fatal helium narcosis. Helium is believed to be the least narcotic gas with a narcotic index of 0.045 (compared to nitrogen at 1.0) and that would make it too dangerous to use around (on the assumption, which may not be valid, of a constant narcotic index with depth) 2,000 meters and uniformly fatal by 3-4 km. Of course, these extrapolations are untested numbers and might not be valid, and no human outside of a submarine or atmospheric suit has gone anywhere near that level.