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There is a roughy 2500x2500x500-1000ft / 750x750x150-300m enclosed subterranean area. The roof is ~250 feet / 75 meters (at most) from the surface. I understand that the thermal gradient will largely not affect the temperature at that elevation and that the temperature would be similar to the outside. It is worth noting that although the area is enclosed it still has access to the surface (ventilation can be employed). There is also limited industry and the heat produced therefrom. Cooling the area is viable, but I would like the area to be ~20 degrees Fahrenheit / 11 degrees Celsius (if not more) cooler than the surface. What reason could exist for a colder atmosphere?

The cold temperature is intended as an obstacle, and as such, would not be convincing if it were manufactured (in world). I also forgot to mention that there is a small lake in the area.

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  • $\begingroup$ Heat would be coming from under the cavern. It'll be about 110F down there. $\endgroup$
    – RonJohn
    Mar 22, 2023 at 16:19

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Natural water cooling

In the ground around the subterranean area an underground river makes serveral loops around the quarry. Water is a good coolant and will transport of the heat. To make the cooling even more effective there could be a reason for the water being quite cold.

For example the underground river could spring from a high mountain top, so that the water is cooled down up there in a normal river before flowing into the underground river.

Another reason for the water to be cold would be if the underground river was supplied by water under big pressure like from the deep ocean (because water under pressure will always be at around 4°C / 39°F depending on minerals in the water). A side effect of high pressure supply would be that the river also has quite some pressure and therefore water would probably be pushed through the stone and into your area, resulting in damp walls and maybe even a small lake.

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    $\begingroup$ This is immensely helpful, water cooling is a great solution, and the area would be near both mountains and an ocean. It can even be a source of usable water for the area, and a necessary price as opposed to a problem that some might want to solve. Thank you. $\endgroup$
    – Sebastian
    Mar 22, 2023 at 9:06
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I mean, this is like your classic 'why is the valley cold' scenario - the cold air that rolls over the lip of the subterranean area will 'fall' into the hole and make it cooler:

Better explanation of this here

So, to keep the inside cool, I would suggest leveraging that phenomena and having the roof designed in such as way as to let the cold dense air roll into the cavern.

Looking at the graph from the link above - a 20 degree Fahrenheit / 11 degrees Celsius differential at night is definitely possible.

You would have to contend with heating during the day - but for that, I think you would just have a highly reflective roof to limit the amount of heating done by the sun.

Other options could be if it's a Quarry (I think it was your question?) - then there will likely be Water around it. Water is really good as it cools on contact - so you could leverage a drainage system that effectively acts as a passive cooling loop - for the inner structure during the day.

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  • $\begingroup$ It was my question, and there is a small lake inside of the area, but the coldness is supposed to be an obstacle, so being manufactured doesn't work well. Thank you for your help, I'll read the article you linked. $\endgroup$
    – Sebastian
    Mar 22, 2023 at 7:07
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    $\begingroup$ What if the opening to the sky was comparatively small and surrounded by high peaks? Cold air could flow down, but the sun would shine in only when directly overhead for a short time each day, and provide little heating. $\endgroup$ Mar 22, 2023 at 16:04
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To prevent people from digging and hide the general outline of the covered area "the organization" built a bunch of solar panels on top and around of covered area.

Solar panels convert, let's say, ~20% of energy, reflect 20 more, use 20 more to heat the air and radiate half of the rest away from earth as infrared light (made up numbers except for SP efficiency). So, the whole area receives significantly less direct sun light and gradually cools down to the new equilibrium. It is unintended consequence and everyone is mad about it but it is too late - "the organization" made a big deal from generating green energy on reclaimed land.

Depending on geology and local climate you might be able to also get a few degrees from geothermal gradient.

https://en.wikipedia.org/wiki/Geothermal_gradient: At depths of a few meters, underground temperatures are similar to the annual average surface temperature. At greater depths, underground temperatures reflect a long-term average over past climate, so that temperatures at the depths of dozens to hundreds of meters contain information about the climate of the last hundreds to thousands of years. Depending on the location, these may be colder than current temperatures due to the colder weather close to the last ice age, or due to more recent climate change.

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Natural ice cave

In climates with cold winter, if a cave has suitable entrance orientation that allows cold air to flow in and then stagnate, it becomes naturally colder than surrounding rock. In summer the cold heavy air won't mix much with lighter warm air above it so the cave stays frozen year-round. Ice will accumulate naturally when there's some water supply.

Some are open to tourists such as https://www.slovakia.com/caves/dobsinska-ice-cave/

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Caves and mines that are deep enough usually have very constant temperature. Mammoth Cave is 54F (or 12C) year round. The temperature in the cave is an average due to the surface temperature, the flow of geologic heat, and the fact that rock is a fairly good insulator.

Close to the entrance or surface the temperature can be quite a bit more variable. You can have things like permanent ice due to snow falling in a hole in the winter, and being protected from the sun in the summer. There is a cavern like this in New Mexico where you can walk down and touch ice in mid August.

But in deep enough caves or mines the temperature will vary only a tiny amount over the year.

The deeper you go the higher the temperature due to interior heat of the Earth. But you have to go down much more than 1000 feet / 300 meters for that to be significant.

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    $\begingroup$ The bottom of OP's cavern is 2500 feet down. $\endgroup$
    – RonJohn
    Mar 22, 2023 at 16:22
  • $\begingroup$ It's 1250 at most $\endgroup$
    – Sebastian
    Mar 22, 2023 at 17:09
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A Dead Planet:

Your world is very old, and drifted through interstellar space for a long time before getting captured by the star it now finds itself around. As such, the core of the planet has cooled to the temperature of background space - very cold. The surface has slowly warmed over the ages, but deep down the world is frigid.

Ancient ice ages:

Your region was very cold for a very long time. Perhaps your cavern even used to be a buried glacier and was gradually melted as the surface warmed. Your cave is thus effectively an ice box, with the surrounding rock keeping it cool.

Intense long seasons:

Your world has regular and very long cycles of long seasons - like decades at a time. The world is currently in a warm phase, but not that long ago it had sustained bitter cold. So the thermal mass of the ground gets cold very deep down, then takes a similarly long period to warm back up.

Evaporating Gasses:

Someone long ago used this cavern, and more beneath it, as storage tanks for vast amounts of ammonia. But the folks who did so are long gone, and the ammonia gradually evaporated (chilling the rock). Further, ammonia from below continues to evaporate and act like a huge refrigerator. There would likely be very high amounts of ammonia gas leaking out of the caverns, making the air rather dangerous (poisonous and flammable, the degree to which is up to you). This would also provide a reason why no one has exploited your caves before - it was simply too dangerous to do so.

Depending on the setting, this could be the reason for your people going there - ammonia could be a valuable resource, and they want to exploit the remaining ammonia in the lower caves for lifting gasses, refrigerant, fertilizer, chemical manufacturing, or water purification.

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