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Surface-dwellers (like us) often went underground for resources e.g. mining. However, I find it hard to find a strong reason (resource or otherwise) for underground creatures to be on the surface. But first, here's the premise:

  1. Use real-life physics and assume a very Earth-like planet.
  2. By underground, I meant deep enough for the creatures to come into frequent contact with magma (so not the typical fantasy dwarves, probably closer to demons). I can't assign an exact number, but I see two extremes: they may few magma the same way we view desert—unpleasant to be in, but survivable, or they may think of magma the way we think of water. Considering that magma is 400–1600°C and thermophiles could only endure 130°C at most, these creatures' biology would be very different. The only hypothetical lifeform I know of that could survive in molten rocks is silicon-dioxide based life.
  3. They are looking for resources, so they don't go up just to set an embassy or something.
  4. The creatures don't have to be intelligent e.g. they may be beasts looking for sustenance.

So what do you think? If we're being general, why would underground creatures venture into the surface? For the specific, what resource would they be searching on the surface?

Here's my own brainstorming on the subject:

  1. They probably don't eat us, since they're likely not carbon-based. If they're silicon-based, their sustenance would likely be composed of silicon. If I were to take a deeper guess, they might be made of silicon, phosphorus, and oxygen or sulfur (drawing analogy to carbon, nitrogen, and oxygen). Phosphorus is more abundant on the surface (or at least closer to the surface), so they might be looking for phosphate rocks.
  2. If they use tools, they probably find metals valuable, though this will depend on their temperature. At the higher end of the temperature, metals we commonly use (e.g. iron, copper) may be too soft. Metals that resist high temperature (eg. tungsten, osmium, iridium) tend to be dense, so would probably be more abundant deep underground. Even if the temperature is low enough for iron to be useful, they would have better luck mining them underground. If they use the really dense metal, they may find meteorite to be an especially rich ore, but they would still be very rare.
  3. A bit cheating, but rather than searching for resources, they might be looking for surface routes to resources. Earth's crust and mantle are mostly solid, so they might find it easier to travel on the surface then drill down instead of drilling underground all the way.
  4. User Li Jun's mention of spice gave me more idea. Silicon-based life may also require salt to live. Sodium and potassium seem to be much more abundant on the crust compared to the mantle, so they might want to harvest them. Interestingly, their close proximity to heat source (magma) might make it economically feasible to mass-produce salt from sea water: just dump them into the magma. A bit tangential, but since our sense of taste evolved to detect the presence of food available around us, silicon-based life probably won't be able to taste sweetness from sugar (a carbon-based molecule), but they might share our sense of sourness (from acids) and saltiness (from salts).
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  • $\begingroup$ maybe plant? leaf,fruit,flower,wood? some of them can be use as herb or seasonal spice. i dont know how silicon dioxide base life deal with plant matter. $\endgroup$ – Li Jun Jun 18 '20 at 11:08
  • $\begingroup$ @LiJun If by spice you meant something to give taste to food, and assuming silicon-based life, then I doubt they would be able to taste our plant. Our sense of taste evolved to detect food that would be available around us. Them tasting sugar would be like us tasting sand. However, your mention of spice did give me more idea, I'll add that to the details. Thanks for the idea! $\endgroup$ – Ray Arifin Jun 18 '20 at 11:22
  • $\begingroup$ btw do the underground has bat ? maybe they need guano or fertilizer or for something ? since i dont know what kind of excrement silicon dioxide base life is. $\endgroup$ – Li Jun Jun 18 '20 at 11:32
  • $\begingroup$ The premise is that they live deep enough that magma is common. I doubt bat could survive the heat. If you're referring to silicon-based analog of bat, then they would probably live deep underground as well, so the underground creatures would have no reason to go to the surface. $\endgroup$ – Ray Arifin Jun 18 '20 at 11:36
  • $\begingroup$ (1) You are assuming that there is magma underneath the surface. The earth's mantle is overwhelmingly solid rock all the way down to the core. This is a very common misconception. (2) Silica-based lifeforms do not exist, and cannot exist. $\endgroup$ – Gimelist Jun 18 '20 at 23:12
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Some options:

  1. Energy - there are not so many source of energy down there and they all are not effective in earth depths. You can't burn oil or coal (for long) - due to products build up. And thermal energy cannot be extracted without good cooler. So they may want to have:
  2. Free water (in form of liquid and vapor) - is perfect source of mechanical energy and medium for different chemical production procceses. There are a lot of water in depths, but all this water is bound in hydrates and hard to extract. So it's actually perfect answer for me: this "demons" know water, know how to use it, it's precious and "demons" know were to get it in vast numbers. It's like us having a ocean of gold at 10 km depths
  3. For less low-tech examples this may be pure metals like gold or copper. Pure metal copper was quite common to find on surface before humans collected it all. But it exists only in menerals forms in-depth. Pure sulfur can also be found only at surface.
  4. In general "salt"-lake minerals, including saltpeter, chalk, and other sedimentary materials. Since they are sedimentary - they all can be found mostly near surface. So this "demons" would very likely to have lime "reverse-mine"
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  • $\begingroup$ Pure metal copper was quite common to find on surface before humans collected it all. — isn't it a weird Earth phenomenon to have metals on the surface? $\endgroup$ – user28434 Jun 18 '20 at 14:01
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    $\begingroup$ Interesting, I've always assumed pure sulfur exists underground since a lot of them are located near volcano. Turns out they only formed when volcanic hydrogen sulfide gas reacted with oxygen. Underground sulfur deposits are also surprisingly shallow. You learn something new everyday. $\endgroup$ – Ray Arifin Jun 18 '20 at 14:06
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Food, clothes and plant-based products

Your creatures might be fond of fruits, berries, nuts, mushrooms, truffles, and veggies that can be found on the surface. Even If they already have a good supply of underground nutrition sources, these still might count as a luxurious dish.

Another use of plants is anything derived from them, for example, natural poisons or clothing. I can't be 100% sure on the clothes since I would have to look up at what temperatures do they burn (fur, cotton), but I've seen a quora post that fluffy cotton can ignite at about 550C, so it still might be a safe pick depending on role of a cloth-wearing creature in the society.

Moreover, aboveground plants and bacterias could be used to develop medicines. The issue is, I'm not sure If they would work or be necessary If your creatures can survive high temperatures. It all depends on what laws and temperatures apply to insides of your creatures' bodies.

There could also be household products like toothbrushes, but again, the answer could be very dependant on your creature's biology and daily activities.

Also, there might be a chance you would like to look at the uses of rubber and resin.

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I find it hard to find a strong reason (resource or otherwise) for underground creatures to be on the surface.

Vacation

Why do we all flock to the beaches every summer? Because it's cooler there, and you can swim in the sea (which is decidedly not our natural habitat). Also you can go out at night and gaze at the stars. It's nice, really.

So it's not that they have a pressing need to be at surface, it's simply a pleasurable experience to them, and a nice break from all this heat and rock.

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Air and Cold

  • Intake air (oxygen), and dump waste gases.

  • A cold reservoir for those heat engines that you need to keep the air and water pumps going. You can mine mountain ranges from the inside for their cold.

  • (Optional) Space for trash disposal if your ecologically-minded folks don't want to dump into magma.

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