⚫ This has been proposed, notably by marine explorer Jacques Cousteau and astronaut Scott Carpenter. It's not going to happen for several reasons.

Permanent housing in water deeper than about 100 ft (30 m) is a bad idea due to the biological effects of pressure, including but not limited to nitrogen narcosis and possible long-term nerve damage, if not talk about the completely unexplored impact of such an environment on pregnancy and young children.

Very little light reaches that far, so seafloor communities rose from the surface to feed. Almost all life in the sea depends on the sun, so whether we live above or below the surface, we continue to depend on the same fisheries and ecology to survive, so living at the bottom of the sea is not at all a fix for overpopulation, if that's a concern. Also, just by staying there on the continental shelf, seafloor communities will disturb the nearshore ecology, likely reducing the overall food supply.

Semi-submersible cities are being explored in some areas (outside of Japan), but they will be high cost, high maintenance, and are not underwater habitats in any real sense. Floating aquaculture facilities can be useful, but they have nothing to do with the issue.

Living underwater is dangerous and expensive. Underwater habitats require completely reliable life support, whether they take in air from the surface or from some other source. A power outage can allow the air to stratify, forming pockets of deadly CO2. Leaks and corrosion will be a constant problem, and constant salt and moisture will wreak havoc on health and equipment. Most oceanic structures have relatively short lives for this reason. So although Europe has buildings that are thousands of years old, it is likely that no underwater structure will be continuously inhabited for more than fifty years.

So while some may enjoy the experience, economics and practicalities will always be heavily stacked against life underwater. Even if we say a large asteroid is coming, it would be much cheaper, easier, and safer to bury yourself underground than to flee underwater.

Finally, humans didn't evolve in cans, and it's already clear that a host of modern ailments, from high cholesterol to myopia to a host of autoimmune diseases, are the result of having locked ourselves in caves of our own making. We need to get out more, not less, and while these and other impacts can be addressed, the easiest way to do it on Earth is to control our population and maintain the opportunity to get out for a regular walk.

colonizing the seas would be an expensive and difficult project due to the corrosive effects of seawater on human construction, the tremendous hydraulic pressures exerted by the water column on the proposed habitat, the shallow water hazards of navigation and tsunamis, and the difficulties for deep water from high pressure leaks and even structural collapse, such as a submarine that has passed "crush depth".

Possible? Maybe. At least hypothetically.

Viable? Probably not.

Unless you keep it very close to the surface, the pressures will make it prohibitively expensive. Even in shallow water, the cost to build and maintain will be a multiple of enclosing the same space on land. There would have to be a very compelling reason to build underwater to justify the risk and expense.

What's more; What would be the point? They would be enormously expensive to build and maintain, and if something went wrong, they could all die. I'm not seeing a silver lining

Most people don't want to live underwater.

There are a few underwater structures, for novelty's sake.

The smaller a structure is, the easier it is for it to withstand the pressure of ocean water. A submarine is easy, a bubble the size of a city would need massive amounts of reinforcement not to fall apart immediately.

⚫ This idea is very similar to the floating cities on Venus;

A manned research station floating in the atmosphere of Venus seems feasible. At about 50 to 54 kilometers from the surface, the environment is quite hospitable compared to the near-vacuum environment in which the International Space Station operates.

For example, the atmospheric pressure at that altitude is similar to the pressure of sea level on Earth. Therefore, the walls of the floating station will not have to withstand a large pressure difference. They wouldn't need to be as hideous as the walls on the ISS. (And not as thick as the walls of a submarine.)

The temperature a little more than 50 kilometers up is in the range of 0 to 50 degrees Celsius. Some air conditioning may be needed, but not the extreme cooling you'd want closer to the surface.

Humans can't breathe Venus' atmosphere, but it contains a variety of elements, including oxygen, nitrogen, hydrogen, carbon, which can be processed into breathable air and drinkable water, and even used to grow plants. Because breathable air is less dense than carbon dioxide, it would function as a gas lift in Venusian conditions, so helium may not be necessary.

It is true that there is some acidity, so the exterior walls and solar panels of a floating research station would have to be made of acid-proof substances. Anyone climbing outside the station would need a supply of oxygen and an acid-proof suit, which would be simpler and less bulky than the pressurized suits required in Earth orbit.

The Soviet/European Vega mission demonstrated that it is possible to parachute research balloons into the atmosphere of Venus and inflate them there. NASA's HAVOC project has been looking at ways to parachute into much larger aircraft: first a robotic aircraft, and then a manned aircraft with a multi-stage rocket module to fly the crew into space again. The idea is that they would then meet an interplanetary transit vehicle in the orbit of Venus.

https://sacd.larc.nasa.gov/branches/space-mission-analysis-branch-smab/smab-projects/havoc/

https://ntrs.nasa.gov/search.jsp?R=20160006329

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160006329.pdf

The astronauts would visit using self-deploying blimps, stay (literally) for a couple of weeks, and return to orbit in their rocket-powered "gondola." From this altitude, they could monitor surface probes in real time, so they could accomplish much more in two weeks than a rover can accomplish in several years.

problem with venus;

⚫ thickening and crushing atmosphere.

⚫ extremely high temperatures.

⚫ acid rain.

advantages of venus;

⚫ minimum terrestrial pressure at cloud level.

⚫ comfortable temperatures at cloud level.

⚫ magnetosphere to block cosmic rays.

⚫ gravity similar to that of the earth.

ISSUE

Take an oven. Seal it. Fill it with gas until it is at a pressure higher than that of the ocean more than half a mile deep, enough pressure to crush a nuclear-powered attack submarine.

Understands? Good. Now fill it with superheated battery acid.

That's Venus.

We have tried to send landers to Venus. They lasted for about an hour or two before cooking, crushing, and dissolving.

Gives a whole new meaning to the phrase "men are from Mars, women are from Venus." Apparently it means that women are tougher than submarines and breathe battery acid while men are comparatively cowardly.

Because of this, landing and surviving a rover would cost much more than sending a probe to Mars. Traveling is not the biggest problem, although landing is.

Venus's own gravity is very high compared to that of Mars, making the descent through the atmosphere a thousand times more difficult. Venus's gravity is very similar to Earth's, and is about twice as high as Mars'. The planet's gravity, coupled with the already superheated atmosphere and high atmospheric pressure, requires extremely powerful heat shields; the most powerful ever built, and they have to work every time.

There are thick clouds of sulfuric acid with violent electrical storms over the entire surface clouding both images and communications. Because of the clouds, we don't know much about Venus through direct contact. We know what we know mainly through radar data.

The extreme heat and lack of visibility also make landing very difficult. Of the 18 landing missions, only 8 were successful. Okay, actually only 15 made it out of Earth orbit, and 2 more partially failed to deploy all components. The longest any lander survived was 127 minutes before losing signal or being destroyed (Venera 13). So we'll put the success rate at 10/15, with a very low lifetime. Even future missions to Venus estimate a run time of one hour (Venera D). The Russians aren't giving up on Venus;

http://en.wikipedia.org/wiki/Observations_and_explorations_of_Venus#Timeline_of_Venus_exploration

It is more practical to have heated capsules on the Martian surface than to have supercooled capsules on the Venusian surface. The main natural handicap any human/rover has in exploring Mars is dust storms, whereas on Venus there are too many to count. That is why we have lost so many probes on the surface of Venus.

Venus' composition might be similar to Earth's inside the planet, but on the surface, it's a very different story. The surface of Venus is made up of rocks that are mostly igneous in nature due to volcanic activity and are extremely alkaline and cannot support life.