This post might seem ridiculous and idiotic to some of you, since it's about nature and cities, society, etc. which I'm sure everyone knows I shouldn't care at all about, being a musician and stuff. But well, I do care, so you should care too. Your own logic slammed right to your face, must hurt a bit, huh? :P But seriously, most people can simply not read this, but if you're in a position that makes you able to forward this idea to someone who cares and can put it into the head of someone who could make it come true, please, please do so. I don't care about money myself, if this would happen, I wouldn't mind not getting money out of being the guy who proposed it, so long as nobody claimed they came up with the whole thing. If such an idea already exists, I'm not aware of it and it's not my intention to hurt anyone's feelings or anything by this. If you have thought about this same thing, feel free to post a hateful spammy comment about that... just know that I'm not going to delete this post just because of that; this is a very general idea, those can't be patented. Or if you somehow have, then, well, congratulations, I guess?
Why don't people build cities under cities? I know, that might sound like a stupid idea but think about it for just a moment. Currently, there are very few (or none at all) people living underground, since everyone wants to go to the highest floor of any building that's ever built and so on, everyone wants to be social, everyone wants to go to forests and see nature, everyone wants to swim, blah blah blah. That wouldn't have to change. There could be an extensive ventilation system that brings air and there could be lots of trees planted in between of houses in the overground parts of cities, just like already is being done in some places. Just a lot, lot more than now. Any plants, even. Maybe have plants from all around the world in every city. Firs, pines, birches, alders, etc. trees on ceilings, vines grown on walls, smaller plants like bushes, etc. on balconies and in the corners of streets, etc. and build houses in a way that would allow uninhabited parts (such as walkways in office buildings) to be filled with forest. Sequoias could be planted in yards, next to houses, etc. and bridges could have edges with support structures strong enough to hold several trees to grow on them, and so on. All that could be achieved quite easily along with a water supply to all those plants by building kind of streets just for that, which would collect some of the water that rains and then release it equally to all the plants (or more to the ones that need more) by just a very, very simple pressure thingy. You know, if a certain amount of water would push a certain hatch, it would open, and have all those hatches have different weights on them so it wouldn't mess up how much water which plant needs. Not exactly complicated.
Now, I'm pretty sure someone is thinking of the problem it would be to maintain, yes, I know. That's why it would all be let to overgrow into one actual forest. It wouldn't grow way too much, because people would use the city, you know. If over a hundred cars drive a street every day, there's no chance it'll get trees on it. If you wouldn't like that much plants, well, you could always move underground! There would be no forest there. Separate gardens, yes, but not actual forests. Those gardens could be built in a layered way around the edges to all "floors" so that they get sunlight and fresh air and stuff, in kind of pits. Also in between of the tunnels, so that it wouldn't be as strictly defined as what's overground and what's undergound to prevent discrimination between people who live in either part of the city and to make it cosier, and more natural too.
Shops, offices, etc. all could be built in the same way underground as they are now overground, hospitals and other "emergency places" could be built at the intersections between the tunnels, etc. for easier and faster access. If you don't know what I mean when I say "tunnels", I mean it'd be all built in a sort of the same way that metro tunnels are, except there would be "houses" for people to live in burrowed to the sides. The actual tunnel parts of the tunnels would be used as roads for cars, etc. and their sides would be walkways, just like there are right now. The transitions from overground to underground would simply be spots that are circular holes which's edges are built with ascending roads for cars and trains, and there'd be elevators and stairs for pedestrians. Those places could also be used for other things that would most likely attract people living in both, such as movie theatres, restaurants, banks, etc. Maybe a couple of hospitals too, and so on. Swimming pools as well. You get the idea.
Anyway, so those living underground wouldn't suffocate (an important thing to remember, isn't it?), there'd be pipes that go all the way from the deepest parts of the deepest underground spot to the highest part of the highest skyscraper overground, where it would go through the air bubble and ta-da, they'd get just the same (well, okay, a little less but who cares?) amount of air. You know, so that the pipe would have a system of pushing the air inside the tunnels. Overground it could all be handled by simply having holes in the air bubble.
...wait, I didn't mention that air bubble before? Well, it would be a gigantic glass half-sphere around the whole city. Or any other translucent material. Its purpose would be to keep pollution inside the city, if that's not obvious. Now, you probably think "but then all the forests would die inside instead of the ones outside", but no, that wouldn't happen; the pollution would be sucked by huge sucktion fan thingies attached to the bubble. Sure, it might not necessarily suck everything, but it would be better than letting all the forests outside (as in the forests we are now polluting) die, since new forests could be grown inside by using the ones outside that would now be a lot fresher and healthier, you know.
Also, the skyscrapers could go through the bubble if necessary for airplanes, etc. so that there'd be landing strips on top of the entire city, supported by those very skyscrapers and everything that is, well, the same structures that would support the bubble; thick beams that would, you know, keep it stable and if the bubble would burst, the city wouldn't be filled with glass (well, it would, but you get the point; it wouldn't all fall down, just some). Also, before you go and think "but hey, man, that would make every city isolated from each other", no, it would not. The roads would still go from one city to another, just not as many and besides, those could easily be substituted with roads that go underground. The bubble would simply be a safety thing for nature. Yeah, it would make traveling more boring, but I mean, c'mon. Would that be too much if it was to keep yourself more healthy and also keep nature more healthy? If you say "yes", you're a selfish jerk.
...and well, some roads underground could be sided with gardens and pools, so it would be just like nature, but not natural. For some that would be a huge thing, but the general population wouldn't give a shit.
Another thing that could become impractical would be energy. Of course, it'd mean a lot more electricity and other forms of energy would really have to be produced/whatever, but that could also come from nature and, yes, I'll say it, the dreaded nuclear power plants. Those should be in the deepest parts of the cities, though, at least a kilometer under the deepest tunnels, to make sure it wouldn't damage the city if it exploded or leaked (unless the explosion would be really, really goddamn huge... like a nuclear bomb was dropped on it (but that wouldn't be the power plant's fault, but the terrorists'...)). And by nature I mean recycling water and stuff. Not to mention that the use of more ecological sources of energy in the first place would be useful, which is something everyone is already thinking about, so that would probably not be such a problem.
But yeah, now comes the problem that always comes: money. It would cost so much that even God couldn't make it possible. (I mean, obviously, He could, but I'm sure He wouldn't. If you're an atheist, well, think of all the presidents and stuff together giving all their money to it or something. That's another thing that could, but would never happen.). If the money problems would go away completely and it wouldn't cost at all (or very little) to do stuff, that would make this idea possible. But if it does somehow get to be an actual plan of the future, I'm sure the important people could squeeze some money for it, obviously, like they always do.