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A mostly autonomous city exists deep underground inside a mountain. They enjoy a modern level of technology and engineering. Other than nuclear power what energy sources could they harness to power their infrastructures?

Maybe geothermal energy? But then where would you dump the heat?

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  • $\begingroup$ Can they employ automated or semi-automated power generation systems on the surface ? E.g. wind, solar ? These would require regular maintenance and occasional repairs, but might be an option depending on your scenario. $\endgroup$ – StephenG Jul 11 at 18:50
  • $\begingroup$ @StephenG I would prefer they interact with the surface as little as possible, as the surface is controlled by a different polity $\endgroup$ – SilverCookies Jul 11 at 18:52
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    $\begingroup$ Heat wouldn't be a problem if you can use air intakes, water intakes, or sink the heat into solid rock (broadly). Nuclear power has the same heat issues, BTW as any other. With air intakes, they could even burn fossil fuels (smoking mountain...) $\endgroup$ – DWKraus Jul 11 at 19:07
  • $\begingroup$ good point, I guess they would need some air exchange at least $\endgroup$ – SilverCookies Jul 12 at 7:45
  • $\begingroup$ Food would seem to be a bigger problem than power generation. Or at least, if they're growing all their food underground, they'll need a lot of power. $\endgroup$ – Nathaniel Jul 12 at 7:59
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Letting the days go by, water flowing underground

enter image description here

https://www.tripadvisor.com/LocationPhotoDirectLink-g641857-d3336030-i71986794-La_Grande_Saline-Salins_les_Bains_Jura_Bourgogne_Franche_Comte.html

Water has been used to generate power since before there was electrical power. There is lots of water flow underground. Depicted - a waterwheel to provide energy in a salt mine. Many existing hydroelectric plants are built into mountains, capitalizing on the gravitational potential energy from melting glaciers and snow as captured in retention ponds.

Energy capture could be completely underground, capitalizing on the natural subterranean movement of water. Of course the water would eventually get back to the surface but your morlock people do not necessarily need to build on the surface to make this work.

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  • $\begingroup$ cool! I didn't think of underground water movement $\endgroup$ – SilverCookies Jul 11 at 15:38
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    $\begingroup$ To add to that: (Coal-)Mines were historically build in a way that ground water naturally flows out of them. The first shafts ("Erbstollen" in German) were dug from the bottom of valleys, slightly angled upwards so that water could flow out by gravity (en.wikipedia.org/wiki/Adit#Drainage). But it did not occur to me that the same water could be used for generating electricity. $\endgroup$ – fho Jul 12 at 13:27
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Maybe geothermal energy? But then where would you dump the heat?

You have to dump heat anyway. The amount of heat given off by animal life in an enclosed space without proper ventilation is enormous. Have you ever been trapped in a lift? I have. We were cooking after about five minutes.

If these are oxygen breathers, they have to have oxygen. Where else but from the surface?

Historically mines have always needed ventilation https://en.wikipedia.org/wiki/Underground_mine_ventilation

They will be at the mercy of the surface dwellers unless they have some valuable resource to sell them or they can somehow hide their ventilation openings.

EDIT

If it is winter on the surface (maybe permanent winter) then what can they sell?

Thermal energy! No way the surface dwellers would harm them then. They're too valuable.

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Coal and oil

If you live underground and you have to dig tunnels in order to expand anyway, why not use stuff that land dwellers have to dig for?

You'll have the advantage that you'll be closer to the reserves of these resources from the start. At least vertically.

As Chasly from UK said, you'll need a connection to the surface anyway. You can send the smoke to the surface to get rid of it.

Gas

And to make myself clear, I mean what comes from the south side of a cow facing north, not processed petrol. In a closed environment every latrine is a gas mine you could tap.

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    $\begingroup$ Ventilation is a significantly bigger problem underground. If they're underground due to political or military conflict, ventilation to the surface is an even bigger issue and a potential deal breaker if they cannot guarantee it is available. $\endgroup$ – StephenG Jul 12 at 3:03
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Underground there are many valuable energy resources. We can use the coal, gas and oil found at the deep of the crust, and we are not reaching the mantle. We can also use the underground water. But still ventilation and oxygen will be a problem, so underground civilizations will require something that makes them able to breathe. They can dig huge tunnels. They also can't acquire food there, as food is found on the surface.

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Air Ventilation/generation.

It's another source of clean energy; if there's cave access of any sort, there's going to be air ventilating through it or even a closed air current that'd circulate around the underground. They could resort to reusing this natural force along with other generators of power.

Perhaps use this method also as a way to vent breathable air into the underground as well. cave systems that are linked from the surface could run equipment to purposely push air into the tunnels; they'd get to harness that as well.

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