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The continent is split apart by a dense area of mountains, covered in glaciers. Any travelers who with to go from one part of the continent to the other would either have to cross the mountains, or go around them by boat, which brings its own set of problems many would rather avoid.

Given the adverse conditions in the mountains, many trails are plagued by frequent snowstorms, much to the detriment of the travelers. Luckily so, the trail goes along several larger cave systems, which happen to be between 15°C and 20°C (59°F and 68°F respectively), which is a boon in comparison to the frosty negative temperatures outside.

The caves are used by travelers to sleep or eat, and some vendors even set up semi-permanent stores to sell goods to those daring to cross the mountains.

Curiously enough, the temperatures are not caused by people trying to heat the cave by conventional means (e.g. camp fires, etc.). Why are the caves in such a high temperature year round?

I've looked into geothermal heating, but I was unable to say for sure if a near geothermal source (e.g. a dormant volcano) was able to deliver such a strong rise in temperature (~20-30 °C) in a way that doesn't have unwanted side-effects (e.g. sulfur gasses in the case of a volcano) .

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    $\begingroup$ What reasearch have you made that were unable to asnwer such, geological, question? $\endgroup$
    – SZCZERZO KŁY
    May 6, 2020 at 13:50
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    $\begingroup$ @SZCZERZOKŁY I've looked into geothermal heating, but I was unable to say for sure if a near geothermal source (e.g. a dormant volcano) was able to deliver such a strong rise in temperature (~20-30 °C) in a way that doesn't have unwanted side-effects (e.g. sulfur gasses in the case of a volcano) $\endgroup$
    – MechMK1
    May 6, 2020 at 13:52
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    $\begingroup$ You could add that comment verbatim into your question as a single paragraph, for future reference to others. @MechMK1 $\endgroup$
    – Nij
    May 7, 2020 at 0:22
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    $\begingroup$ @Nij Yes, I should have added what research I have done prior. I honestly don't even know why I didn't do that in the first place. $\endgroup$
    – MechMK1
    May 7, 2020 at 7:25
  • $\begingroup$ Is there something such as heated magma or the like inside the mountains, broken only by caves. I don't know how this would work, but it migh make sense. $\endgroup$
    – Zulop Zig
    May 7, 2020 at 9:24

9 Answers 9

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Geothermal heat.

It's well established that the more in depth one goes, the higher the temperature is. It It is called geothermal gradient

Geothermal gradient is the rate of increasing temperature with respect to increasing depth in Earth's interior. Away from tectonic plate boundaries, it is about 25–30 °C/km (72–87 °F/mi) of depth near the surface in most of the world

If you are close to a tectonically active area, like Iceland, that gradient is even higher.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch ♦
    May 8, 2020 at 2:30
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Caves are that way.

Caves are well insulated. Cave temperature equals average outside temperature for the year.

https://startcaving.com/info/heres-why-caves-stay-the-same-temperature-year-round

Why do caves stay at the same temperature year-round? Caves stay at the same temperature year-round because they are thermally insulated from the external substances that flow into them. The temperature of the air and liquids that flow into caves has little impact on the crust that forms the cave, which has a much larger thermal capacity than liquid or air.

While there are some exceptions that may make the cave seem warmer or colder than expected, such as depth of the cave, distance from the mouth, and number of openings into the body of the cave, it can generally be accepted that a cave will retain a yearly temperature about equal to that of the average annual surface temperature of the region in which it is located.

Your caves are large. It is cold higher in the mountains but warmer in the summer. At the base of the mountains it is less cold in winter and hot in the summer. The cave spans this whole area.

You do not need lava pools or radioactive decay to keep your cave at a constant 15F. Most caves in temperate areas are just that way because they are caves.

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  • $\begingroup$ "...At the base of the mountains it is less cold in winter and hot in the summer" ... just for the same reason inside the caves it will be colder at higher levels. Well, I am not sure how much. $\endgroup$
    – fraxinus
    May 7, 2020 at 8:46
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    $\begingroup$ The reasons for air being cooler at higher altitudes wouldn't work in a well insulated enclosed space. In an enclosed space warmer air tend to accumulate on the ceiling. $\endgroup$
    – Pere
    May 8, 2020 at 10:45
  • $\begingroup$ 15F is below freezing. Many caves average between 50 to 70F. $\endgroup$
    – candied_orange
    May 9, 2020 at 17:30
  • $\begingroup$ @candied_orange - that pesky imperial system. good thing I am not sending orbiters to Mars. $\endgroup$
    – Willk
    May 9, 2020 at 18:13
  • $\begingroup$ Cute. How about an edit? $\endgroup$
    – candied_orange
    May 9, 2020 at 18:17
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A Natural Nuclear Reactor

Natural Reactors existed on Earth a few billion years ago, in places such as Oklo. Slightly permeable rock containing uranium ore, when saturated with water, became critical for about 30 minutes, heating the water and driving it to steam. After that 30 minutes there wasn't enough water to sustain the reaction, and it stopped reacting for about 2 1/2 hours. This cycle continued for about 150 thousand years. A reactor of this type can generate about 100kW of heat, and the surrounding rock both shields from radiation and evens out the changes in temperature.

This type of reactor was possible when the natural concentration of U-235 was higher, those billion years ago. It would be relatively safe to occupy this cave, although you might not want to drink the water.

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    $\begingroup$ Very interesting spin on things, especially given the "Actually, this is real" angle $\endgroup$
    – MechMK1
    May 6, 2020 at 16:40
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Hot springs

Your caves have geothermal springs in them, which not only keep them warm - they keep the caves expletively hot in the lower places. From the wiki:

A hot spring, hydrothermal spring, or geothermal spring is a spring produced by the emergence of geothermally heated groundwater that rises from the Earth's crust. While some of these springs contain water that is a safe temperature for bathing, others are so hot that immersion can result in an injury or death.

An example of a cave with a hot spring is Grjótagjá (probably named after a Great Old One), in Iceland. The waters inside the cave are very hot, and between 1975 and 1984 its temperature had even rose above 50C/122F.

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    $\begingroup$ This seems like the most plausible answer for the setting OP has given. It has been completely normal for people to setup shops or baths or little towns around hot springs throughout human history. Especially in a colder mountainous region this would make perfect sense as a common traveler's stop over. $\endgroup$
    – TitaniumTurtle
    May 6, 2020 at 14:48
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    $\begingroup$ People are even known to name the town something appropriately imaginative, like ... Bath. $\endgroup$
    – user_1818839
    May 7, 2020 at 20:47
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    $\begingroup$ With hot springs you avoid all the nasty side effects of volcanism, such as sulpur, just like the OP stated. If the hot water surfaces, sub-surfaces?, somewhat distant from the cave, it can loose any dissolved gases without polluting the caves. $\endgroup$
    – Dohn Joe
    May 8, 2020 at 11:26
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Wibbles

Wibbles are small fauna which ingest carbon dioxide and excrete oxygen, much like plant life on Earth. But they also ingest potassium salts which are found in abundance throughout those mountain ranges. Wibbles have a very high metabolic rate and therefore constantly emit heat. They tend to favour caves because they don't like cold or daylight, and also because they are very slow-moving and can be predated by various avians which inhabit those mountain ranges.

Since the inhabitants of your world have not yet entered the scentific age no-one can say what happens to those potassium salts: it may be that spending time in those caves does severe long-term damage to the bodily organs of the inhabitants of your world.

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  • $\begingroup$ Some weird metabolism they must have, welcome to worldbuilding. (From review). $\endgroup$
    – Escaped dental patient.
    May 6, 2020 at 21:26
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    $\begingroup$ This also takes care of making sure there is a safe level of oxygen inside the caves for the travelers. $\endgroup$
    – manassehkatz-Moving 2 Codidact
    May 7, 2020 at 3:34
  • $\begingroup$ I wonder if a large concentration of wibbles in a cave with poor airflow leads to...explosive potential. $\endgroup$
    – user3067860
    May 8, 2020 at 12:15
  • $\begingroup$ This has some serious scientific plausibility problems. Ingesting carbon dioxide and excreting oxygen is an endothermic process. Plants do it only when they have sunlight to provide the energy; at night they absorb oxygen and excrete carbon dioxide. Your wibbles must be eating something very energetic and breaking it down in order to produce both oxygen and heat. Maybe the potassium salts could qualify, but salts usually aren't high-energy compounds. Also, animals with a high metabolic rate are generally not slow-moving. $\endgroup$
    – zwol
    May 9, 2020 at 15:28
  • $\begingroup$ I'm not vouching for plausibility. I'm merely saying that that's what happens on that world, with that species. Wibbles have some other barely believable characteristics: for example, certain subspecies migrate for thousands of km every 13 years in a ever-decreasing spirals, in the contrary direction to the planet's spin, and no-one knows why. Under 1% survive this process. It is thought by the way that although they could move fast, they choose not to. Again, no-one can say why. $\endgroup$
    – mike rodent
    May 9, 2020 at 15:33
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Granite is radioactive

Granite is naturally radioactive and gives of heat (as well as radon gas).

Granite caves would naturally be warmer with no obvious heat source

Heat Production of Granite

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  • $\begingroup$ Hey, great idea! How radioactive would the granite have to be? I’m only seeing 2-8 microwatts per cubic meter in the study. $\endgroup$
    – IronEagle
    May 8, 2020 at 8:13
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    $\begingroup$ The major problem with this is the "as well as radon gas" part. If you had enough granite around to sufficiently heat a space, the radon would likely be at lethal levels. (Note: I am not qualified to do the actual calculations on that, it just seems like a serious problem that would need to be overcome somehow) $\endgroup$
    – Kevin
    May 8, 2020 at 15:11
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Conferences of theologians

All those caves are completely stuffed with people, all the time. Most of the time you have to squeeze past people quite assertively to get inside them.

That's because the people of that world believe caves, any caves, are particularly spiritual places, and since religion and matters spiritual dominate the lives of all the people on that planet, it means that conferences of theologians are held in every single cave, even the smallest, on an almost-permanent basis. 99 times out of 100 they are discussing some aspect of the doctrine of transubstantiation.

Most of these monks, nuns and assorted shamans believe that they must live lives of poverty; consequently they dress in very thin material. In the mountains, where you find the caves, they therefore have to bring stocks of firewood, which they burn inside the caves and at their entrances, producing acrid smoke and making the general air of fractiousness and discomfort even greater.

Those caves drive everyone mad. Everyone wants everyone else not to be there. But all this humanity/alienity and burning wood keeps the caves uncomfortably hot, all the time.

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    $\begingroup$ Good idea, but the caves would soon become unpassable due to the constant holy wars being fought in them. Not just between followers of different gods, or different pantheons, but between followers of the same god(s) who pray to them differently - meaning that everyone is wrong! SMITE the unbelievers! Send them all to perdition eternal for their recalcitrant way of saying "Ohhmmmm!" at the end of a prayer instead of the HOLY AND PROPER word, "Mizratooty!"!!!! (Kids! These people are professionals!! Don't try this at home!!!) $\endgroup$
    – Bob Jarvis - Слава Україні
    May 9, 2020 at 19:22
  • $\begingroup$ It's funny you should mention holy wars (although I'm not saying holy wars are funny). In fact civilisation of some kind or another has existed on this planet for hundreds of thousands of Earth years, although the current inhabitants have virtually no interest in technology of any kind: they despise everything relating to technology, and whole schools of thought debate whether using wheels is ever acceptable (most say no). But one, to us, wonderful characteristic of this planet is that physical violence of any kind is totally, I mean totally, unknown. If a toddler ever lashes out at another... $\endgroup$
    – mike rodent
    May 9, 2020 at 19:27
  • $\begingroup$ ... they are meant with gales of laughter, and adults rush in to stop it happening. Instead of violence they employ derision and insults. As everyone knows the insults will never lead to violence these can be, and are, very, very extreme, and often a single insult can last for days. This complete absence of violence is almost certainly an anthropological consequence of sectarian, religious and doctrinal wars fought many millennia ago. $\endgroup$
    – mike rodent
    May 9, 2020 at 19:30
  • $\begingroup$ GALES OF DERISIVE LAUGHTER, BRUCE! :-) $\endgroup$
    – Bob Jarvis - Слава Україні
    May 9, 2020 at 19:49
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Radiators

Tens of thousands of years ago a highly advanced civilisation spent 100s of years installing electric radiators in all these caves. They are powered by wires which lead off into the bedrock... and the source of the electrical energy is unknown.

The present civilisations on that world are pre-scientific, or more accurately post-scientific. They believe they can account for the powering of the radiators, but since all their explanations are borne of arbitrary supersition, and none has been proven in any way, none of them is of any interest.

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  • $\begingroup$ ...and six months after the highly advanced civilization collapsed into anarchy (see the "Conferences of Theologians" answer elsewhere) all the wires were ripped out by teenaged hooligans looking to sell the copper to buy intoxicants. Because civilizations may rise and civilizations may fall, but teenagers stay the same... $\endgroup$
    – Bob Jarvis - Слава Україні
    May 9, 2020 at 19:30
  • $\begingroup$ I don't mean to be rude, but have you ever actually been to this planet? Hearsay is one thing... On this planet the radiators' copper wires are regarded with unparalleled awe (one reason why the caves are held in such sanctity). Not least because over the millennia they have frayed and been repatched so many times that people frequently get electric shocks from them, which they naturally interpret as displeasure on the part of the numinous spirits who jealously guard these holy places. Sweaty, sleepless, irritable theologians would also stop any such ill-intentioned teenagers. $\endgroup$
    – mike rodent
    May 9, 2020 at 19:37
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They're deep.

These caves may be as deep as you (the world-builder) wants them to be, and deeper caves are warmer.

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  • $\begingroup$ what does this add that the accepted answer doesn't already say (and in more detail)? $\endgroup$
    – TylerH
    May 8, 2020 at 13:47
  • $\begingroup$ Simplicity itself is a virtue in an answer. Especially when a questioner ties himself in knots with a convoluted question that has an extremely simple answer. $\endgroup$
    – workerjoe
    May 8, 2020 at 14:26
  • $\begingroup$ How would very deep caves lend themselves to being good as shelters for travelers and for semi-permanent shops? If people have to travel into a very deep cave that will take a lot of time and be potentially dangerous $\endgroup$
    – Kevin
    May 8, 2020 at 15:12
  • $\begingroup$ They could angle down at a gentle slope for a couple of miles. $\endgroup$
    – workerjoe
    May 8, 2020 at 15:31
  • $\begingroup$ @workerjoe walking a couple of miles in a tunnel to get to the warm part doesn't seem to fit the desired convenient trailside shelters. $\endgroup$
    – barbecue
    May 9, 2020 at 16:29

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