70% of the earth is covered by ocean. It's wet and salty, and only really populated with life in the shallows near the continents. Further offshore in the deep, life lacks the shelter to survive, so there's just an endless expanse of water. On the depths below, there is darkness, as the ocean has absorbed all light, and a sand-and-rock bottom. I have a plan to make this...useful. I wrote about it once before, so let's see
Using construction submarines, we build a large and hollow structure, and use cans of strongly compressed air to purge it of water. The structure will need a semipermiable membrane filter, to filter seawater into drinkable water, a submarine airlock, so people can reach it, and be divided into rooms. This is now an undersea city. It can be sold to the nearest nation....or an exploratory sort of person to found a new one, if it's far enough away from the others. The city will need electric power for lighting, some sort of commerce or hydroponics for food, and some plan to deal with the brine and wastewater. (They could be taken to treatment plants onshore with pipes, or they could be just flushed back out to the ocean at large.)
A better still idea would be an underground tunnel at the bottom, below even the surface, with rail and road lines that lead to the nearest on-land nation. Better than that would be to connect to the nearest several, thus becoming a transit hub. Air would flow through the tunnel, as would trains and cars carrying goods and people. The self-sufficiency of the sea city would be greatly improved. And you wouldn't need the expensive and difficult submarine lock anymore.
Last time, I suggested windows. These would be, from the point of view of the inhabitants of the city, useless. In the kinds of locations where this would be most plausible, there isn't enough light to see anything. It's dark 24/7, because the ocean above absorbed all the light that falls on it. Windows would only be interesting ecologically, as there could now be an ecosystem that starts with plants living on the light that leaks out sea city's windows. What plant could endure the pressure? Not many. The windows would be as helpful as your house's windows are on a dark and moonless and cloudy night. You can see your own reflection...and not much else.
I predict this will gain little traction until it becomes necessary. Already, every scrap of land on Earth is claimed by some nation or other, but many nations have large empty tracks. If these fill, perhaps offshore colonies may be considered, but no one's going to bother before. Perhaps a bunch of wealthy libertarians might like to do this, as it would leave them free of existing governments and rules as well as being hard to invade. Perhaps a displaced group might want to find some place, any place, where they won't be persecuted, and this is one of many possible solutions. Perhaps a group fond of solitude might do this to shut themselves off from the rest of the world.
Nah, not likely.