Human beings have gotten all the way to the moon, but can’t get far under the sea. In space we can wear special suits to decrease effects of pressure change and deliver air to breathe, but (in brief) the farther you go down underwater, the more these same conditions constantly change in difficult ways.
For ordinary scuba diving, divers wear a tank of regular air. A compressor squeezes ordinary filtered air into the tank, so that several times more air fits. For more time underwater than a tank allows, a diver can breathe from a special long hose from the surface. How far you can go depends on the length of the hose and the power to compress the air to the right pressure. With other specific training, you can wear a rebreather. A rebreather scrubs and reuses exhaled air instead of losing the exhale into the water (shortest description). In all these cases, the deeper and the longer you stay, the more nitrogen in the air you breathe dissolves all over your body.
When a diver starts back to the surface, pressure reduces all the way up, letting nitrogen back out. You need to come up slowly enough and not have stayed too long to be able to go directly to the surface without the nitrogen forming bubbles inside your body, part of the diving injury called decompression sickness or The Bends. Decompression sickness, and bubble formation, transit, and medical effects was a passion of my career work in physiology for many years. Still is.
On deeper dives, it works better to breathe less nitrogen. You can’t substitute more oxygen at deep depths, because oxygen becomes increasingly toxic. You need a gas that doesn’t make as much trouble during each depth and time. One choice is helium to replace some or all nitrogen, and part of the oxygen.
If you have lots of dissolved nitrogen or helium or other gases chosen for a long and/or deep dive, you need to stop on the way up, called a decompression stop. Where and when and for how long to stop is interesting, and the subject of research and arguments (discussions) among scientist and divers. Different Navies and commercial companies use different protocols, some known well, some closely guarded as company secrets.
For extreme depth diving for research, commercial work like oil drilling, mining, and communications, military surveillance, espionage, and “proprietary commercial interests,” divers can spend time on “deco” stops, but for long dives, many stops are needed, some more than 10 hours. Doesn’t work to do that, then go back to work the next day and repeat. One solution is to stay down inside the rig or habitat or other enclosure designed for that.
At each depth, you can only absorb a certain amount of gas. After that, no more fits. It doesn’t matter how long you stay past a certain point, you have the same decompression obligation on the way up. Staying down until you are full of gas is called saturation diving. You can stay down a week or a month, then decompress once. Decompression can be done in the water, but there are problems of cold, darkness, bathroom needs, and gas supply. Another solution is inside a vehicle designed for that purpose. The decompression vehicle can be raised and removed from the water, and the divers inside slowly decompress safely. It was also experimented, to drag divers straight to the surface and throw them as fast as possible into a surface chamber to quickly compress them back to pressures at depth, then slowly release according to algorithms people back then decided were right. Tragically, some regular scuba divers heard about these two kinds of “surface decompression” and thinking it meant the water surface, managed to publish articles in diving magazines, and give lectures at dive shows, with that misinformation being widely repeated, that one could come straight to the surface after deep dives and float around in an inner tube and read dirty magazines, as the guys in the special recompression chambers did to pass the couple days they’d spend.
You wouldn’t turn inside out from the huge pressure differential produced, as depicted in some science fiction movies, but it might kill you as effectively. That is a sample of what happens when reporters don’t read what scientists write in their research articles, just repeat some sentences taken out of context, conclusions in the abstracts, or what someone else wrote.