(Please bear in mind I am quite new here, and I have in fact created this account just now. I've tried my best to follow all the rules I've read browsing around. I do realize this is quite a broad question.)
I'm currently in the brainstorming stage, so I am completely open to drastically changing elements of the lore. I'd like to try to be as realistic as possible, which is why I thought of asking. Because, well, I don't really know much about certain things.
Let's set our setting first shall we. Our story is set hundreds — perhaps thousands — of years into the future, after global warming and climate change have taken all their tolls on the Earth. We're following a realistic timeline, by the way, so perhaps the Earth went into chaos very slowly over a few centuries starting from the Industrial Revolution. Basically everything goes as expected in an extreme climate change situation (which is why the story takes place so far into the future), but I'm assuming certain factors that would arise as a result of this event.
I know this might be deviating from the question a bit, but I think it would be important to take note of the surroundings before proceeding with the actual underground colony.
- Temperature. Global temperatures have risen by, well, a lot. The tropics have far grown too hot to accommodate any long-term settlement, and so virtually all of the global population lives within the Arctic Circle. Not to say they can't live further south, but it's always better in the north. Let's assume temperatures here in the Arctic are around 30 degrees Celsius (86F), which means that the Arctic has more or less assumed a climate similar to what the tropics do experience today.
- Water. As a result of the ice melting, water is very much abundant. Whether it's freshwater, saltwater or contaminated water, well, I do not know. Massive storms are already common in the Arctic and Antarctic — you would therefore know what to expect down south in the tropics.
- Topography. Now this one is a bit unrealistic. I'd like my world to look much like a wasteland, bare dirt with a few small plant and shrub colonies here and there. Deserts are also expanding quickly (I read somewhere that that would happen eventually, but I don't really know). The goal is to make my world look as unappealing as possible on the surface, which justifies having humans live completely isolated underground.
With that in mind, here are some factors I would consider in building this underground society. Again, I'm open to all sorts of revisions as long as the main gist of the story holds true. For a lot of factors, I'd imagine they aren't constant all throughout the board, so I do take note of that.
- Civilization. Progress has slowed to a halt. The humans are still there, still humans, but only in very few numbers (Preferably as realistic as possible). This is where our whole underground thing comes into play — the remaining humans have dug intricate tunnel complexes known as "Bunkers", most of which are located in the far north (say Russia, Canada, and the like). Communications and long-distance travel have disappeared; basically, nobody is connected to anyone except their own colony. Formal, organized government has also mostly disappeared; the bunkers are kept running by the individual efforts of its inhabitants. Take note that humanity's only goal is to stay alive. People have more or less stopped large-scale conflict, money and economy isn't really much of a thing, and they all generally get along well (Reiterating the fact that they only know the people from their own bunker). The bunkers themselves are rather disorganized; not luxurious, rather dystopic, but enough to keep everybody alive and well. Of course people do cooperate on a large scale when needed, such as during disease outbreaks and other emergencies.
- Culture. Just inserting a little note here — I wouldn't want to expound on the effects of having people of different nationalities get together. For the sake of simplicity we'll assume everyone in the bunker has developed more or less a similar culture and language (English?) through the course of time.
- Location. We're focusing on a single colony, located somewhere along the outskirts of (what was once) Yakutsk in Russia. Yakutsk is located roughly in the center of Siberia, and is very much isolated. It's built near the Lena River which flows into the Arctic, and from what I've read it's a pretty large river so we can safely assume it has grown to a huge size after all the ice melted. Take note that the entrance to the bunker is located near the bank of the Lena. Now, I said earlier that temperatures have risen to around 30+ degrees Celsius here, so Siberia is basically not Siberia anymore. Yakutsk also happens to be at the forefront of the expansion of the Gobi Desert, so we have sand dotting the barren soil here and there.
- Depth. I should probably note here also that the bunkers aren't very deep. Ballpark figure, perhaps in the few hundreds of feet below the surface on average. Certain facilities can go higher or lower as needed. The surface isn't hostile enough to justify digging kilometers down, but it's better just not to be up there. Perhaps the poorer classes would live closer, maybe even a hundred or so feet below?
- Water. Now we know the massive Lena River is just next door, but it's on the surface. Is it plausible for our dwellers to gather a fair amount of groundwater without surfacing? I know there are subterranean rivers and such, but how lucky do we have to be to find a large underground river that would sustain the colony for hundreds of years? Or do we just build facilities that collect water from the aquifers below the river?
- Power. Again, the main rule here is no surface, so solar power is instantly ruled out. Could geothermal power be the solution? Again, how lucky would we have to be to find both underground water and a heat source for electricity? Or could we manipulate the water in some way, like in a dam?
- Ventilation. Alright, fine. For this one, I'll let them build air ducts and filtration systems to the surface. But these were built long before the surface itself became so unappealing. Also, given the appropriate facilities, would it be easy to distribute air and cooling throughout the entire complex?
- Food. Mostly plants grown underground, with all the new high-density farming techniques we have today. I heard artificial plant lighting is a thing now so with electricity and water out of the way (hopefully), I'd assume there would be no problem with it. Animals would also be cultivated along with the plants, some just for the sake of preservation and others for consumption.
- HR. I don't think this would be much of a problem. Let's say everyone thinks rationally and knows their responsibility in society. Miners to dig new tunnels and just expand in general, farmers and agricultural workers, industrial workers creating the equipment, people manning the utilities, doctors, all that. People also reproduce as needed (perhaps, sometimes, as desired), but of course the growth rate is terribly slow compared to what we have today. I'd imagine the population of each bunker colony to be around a few hundreds to thousands, but again I'm still brainstorming so that could change. Entertainment and psychological well-being is a given, so we can ignore that.
Well, that was much longer than expected... You could say I do need a lot of help on this undertaking of mine. I'm not demanding or expecting answers for everything, of course. But any help is definitely greatly appreciated. Thanks for your time!