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:
- Use real-life physics and assume a very Earth-like planet.
- 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.
- They are looking for resources, so they don't go up just to set an embassy or something.
- 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:
- 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.
- 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.
- 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.
- 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).