It's called the bends. It's actually a generic term like DCS (decompression sickness) that describes a few problems that a diver can experience, when ascending from depth. It can take the form of a simple skin bend (bruising under the skin) to a full blown embolism or the brain or lungs. Essentially, it happens when gasses absorbed during respiration, under pressure, aren't allowed to be respirated back out of the body fast enough. These gasses grow in size as you come up, further crippling the bodies ability to get rid of them. You can die from a brain embolism almost immediately. Lung embolisms are dirty affairs. You won't be able to breathe properly and will likely die by dronwing in your own blood as the individual air sacs in your lungs explode from the pressure differences. They also cause large bubbles to form, blocking the gas exchange of lung to blood. If you didn't embolize immediately upon surfacing, it may still happen within a few minutes. You can also die a painful death with gas trapped in your joints. Your blood can actually look like a shaken can of soda. Your heart doesn't like to pump foam, but liquid blood. Heart attacks at this moment are common. This is actually where the term "bends" came from. Cassion workers and divers came up from the depths and bent over double in pain. Decompression (not depressurization) is a method we use, to allow time for those gasses to leave us. You ascend slowly to predetermined deco stops. Sit there for whatever time is needed to gas off and continue your ascent. Your deco stops and time are all a function of how deep you've been and how long you spent there. The deeper and longer, the more frequent stops you need and the longer time required at each stop to get rid of gasses that your body has absorbed, but not used. A decompression chamber works by the same principle, of letting you slowly off gas. A diver in a wet suit can dive as deep as he wants. The record currently is over 1,000 feet. The deeper you go, the more gas you'll need to help decompress. It'll also depend on the gas you're using. If you're on Nitrox (enriched air) you can't go below 120 feet without suffering from oxygen toxicity ( it kills you dead in an instant), on regular compressed air, you can hit 250 feet, but will be narced out of your head. On Tri Mix, you can cut a custom gas mix to go as far down as practical. It's pretty cold past 200 feet though and he'd likely want to use a dry suit. I've never gone past 150 without one. By the way, don't confuse the bends with nitrogen narcosis. They're primarily caused by the same gas, nitrogen, but they are different animals. Narcosis is the effect that nitrogen has on the brain, when breathed under pressure. It's like having a martini for every ten feet, after about 120 feet, on average. You get pretty silly. I know I have. It goes away the moment you get back to around 120 feet. Some people are more suseptible than others.