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I have already taken into concern renewable sources and several other factors.

Ventilation

Oxygen easily gets into this underground system via diffusion and likewise CO2 easily goes out. Plants also draw CO2 from the atmosphere, facilitating diffusion of CO2 from high concentration(underground) to low concentration(atmosphere) Oxygen goes the opposite way again via diffusion. And this keeps happening. The only way these people get hypoxic underground besides lung and heart issues is if tunnels collapse on them. This tunnel collapse leads to injury and in some cases hypoxia.

But is this plausible that plants on the surface draw enough CO2 that diffusion is enough to get CO2 out and O2 in? Will there be some point underground where CO2 just builds and builds(a poison zone if you want to put it that way)?

Basically I am asking if there is some point where machinery(small if needed) or some other process besides photosynthesis is needed to facilitate gas exchange between atmosphere and underground Could making a chamber for plants with lots of light underground get rid of this poison zone(or if not, move it down further)?

Now here is some planet data that I know for sure:

Radius: 2 earth radii

Mass: 4 earth masses

Gravity: 1G(earth gravity)

This may or may not be needed to answer this question. I think it might be needed in order to know the depth at which this poison zone would be assuming a constant atmospheric CO2.

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    $\begingroup$ I'm struggling to understand what the actual question is. Perhaps you could also tells us more about the underground city? Also, why are you talking about plants, how do you have plants photosynthesizing in an underground city? If you have plants everywhere, then you need skylights, so your city isn't really underground as much as in a hole. $\endgroup$ – kingledion Sep 19 '16 at 15:24
  • $\begingroup$ I am talking about plants because these draw CO2 out and give oxygen more than they use oxygen. With less CO2 in the atmosphere, the underground CO2 should have an easier time going out of the underground city. Any visible light will do for growing plants(though UV does help) Likewise oxygen should easily flow into the underground system again due to plants. By plants everywhere I don't necessarily mean trees and vines everywhere. In fact, most likely where they would build a city is a nice open grassland with forests close to but not in the city. $\endgroup$ – Caters Sep 19 '16 at 15:29
  • $\begingroup$ @Caters - Mass and gravity are tied to one another, 4x mass and 1x gravity doesn't work. Do you mean Volume/size = 4 Earths? $\endgroup$ – Twelfth Sep 19 '16 at 18:53
  • $\begingroup$ But gravity is also tied to radius and on much larger scales, distance. $\endgroup$ – Caters Sep 19 '16 at 19:09
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    $\begingroup$ Two times the radius = eight times the volume. What is your planet made of?! Something half as dense as Earth? Surprisingly, this indeed would give 1g. $\endgroup$ – Mołot Sep 20 '16 at 10:50
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I really don't think diffusion will do it for you. Our lungs work because of their vast surface area, and unless you plan to have a bowl-shaped city (basically just a hole) you're not going to have enough surface area for adequate air exchange.

Tunnels are problematic if they collapse, but with a few simple precautions, the risk of collapse is minimal. First, make your air tunnels as vertical as possible. Don't slant, don't slope, just go straight down. That way, if there is an earthquake, the weight above the tunnel won't collapse it because there is little to no real weight pressing down on the tunnel. Second, wall the tunnels in concrete. If your society is not that advanced, use anything. Even resin or baked clay, just something to keep dirt from falling off the walls and into the tunnel. If the city is build into bedrock, this isn't really a problem. These shafts can be dug to any depth you need, so your city can go down as far as you like without too much degrade in air quality as long as you have enough shafts.

With several, large, vertical shafts in place throughout the city, install fans. Set up some shafts with fans pulling fresh air down, and some with fans blowing old air up and out. Keep canaries around the updraft shafts. The air quality will be the worst there, so if the birds are fine, you should be too. If the birds die, move the cages farther away. Establish barriers to danger zones based on bird tests. All the fans should be alarmed. If any one stops working, a loud alarm will sound, warning people that there is a problem and that they should seek better air. I also recommend CO2 and CO monitors, much like the ones we have today. These should be installed around the city and should be linked to a central server so that people can be warned that an area is unsafe.

I wouldn't count on plants to oxygenate your city, but if you really want to use them, you have some real-world examples to draw from. http://www.pbs.org/newshour/art/grow-plants-underground/ If nothing else, you could have a plant room for emergencies, just in case the fans stop or the shafts collapse or something.

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Make the air shafts wide enough (a city at the bottom of a conical pit), and the ventilation would be the last of your problems. Make the shafts small enough, and you get a death trap at the basement level.
Also, the result depends on atmospheric composition and pressure, temperature, underground temperature, winds at the surface, etc.

So, unless you're into lots of physics and math, just handwave that the air shafts are wide enough (or not wide enough, if you'd like) to sustain the city.

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  • $\begingroup$ You see, I wanted to try to avoid that since if there is an earthquake, those along with tunnels are going to collapse completely. Tunnels will cause hypoxia via aspiration and air shafts would cause hypoxia via hypercapnia(in other words it isn't an oxygenation issue but rather a carbon dioxide issue that leads to problems with oxygen). There is already enough risk for hypoxia in the tunnels if they collapse as is. In fact this risk of collapse is why I have the homes and buildings chambered in the first place(chambers lower risk of collapse) So I would prefer to keep the collapse risk low. $\endgroup$ – Caters Sep 19 '16 at 17:30

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