I am writing a story set on a mining colony moon with some very odd geological conditions, and while this idea is admittedly likely impossible to fully make geologically plausible but I wanted to see if there was some way to at least make it sort of plausible

The story is set on a moon that is tidally locked to a gas giant, along side dozens of other moons, all causing extreme tidal forces on the crust, similar to Jupiter's moon Io. Unlike Io, I want the moon's surface to be earth-like, capable of supporting breathable air, liquid water, plants, animals, and of course humans. However, once you go more than thousand or so feet beneath the surface, the tidal effects of the gas giant mean that the ground is undergoing constant rapid churning, with the sort of geological activity that takes thousands or millions of years on Earth happening in days or weeks. The underground is very mineral rich due to the volcanism, very porous, filled with large naturally occurring caves with breathable air, and supports its own parallel eco-system of plants and animals, including predatory megafauna, that were at some point churned under from the surface and then continued evolving to adapt to life underground.

The livelihood of the colony revolves around mining highly valuable minerals, and this process works sort of like ice fishing. The miners living on the stable top layers of earth dig tunnels down to the turbulent deeper layers, set up temporary underground boom towns in the caves with hundreds of miners working against the clock to strip everything they can out before the tidal forces move the minerals deeper underground and the miners with them. When it becomes too dangerous to stay, they haul the whole boom town back up to the surface, send the minerals to be refined, and by then a new trove of minerals has moved underneath their "fishing hole", and the whole process begins again. This cycle can sometimes last months, sometimes only a few days, and it can be very unpredictable how long a mine stays stable before it shifts back to lower layers. This is all obviously very dangerous, not only because of all the usual dangers of mining we see on Earth like flooding, fires, bad air, etc but also because the underground has large predatory creatures, and of course the constantly changing geology that can bury the entire expedition alive.

So that's the setup I want; how can I make it more realistic to actual geology and astrophysics and such while still keeping the core idea? I know there is the example of a highly geologically active moon in Io I mentioned, from what I have read even the quick moving Io is not nearly this quickly moving. Not to mention that Io is a volcanic hellscape and not at all Earthlike on its surface, since all the geologic instability means tons of earthquakes and volcanoes that would make life on the surface impossible. So I am wondering what are some ways that these sorts of geological conditions can exist while still allowing for the moon's surface itself to stay in the same place and be geologically stable and habitable?

My best theory so far is the idea of the surface settlements being mostly built on areas composed of a rock that is much sturdier and more durable than the earth underneath and sort of floats over everything else, allowing largely earth like lifestyle with the only concerns being a relatively high frequency of earthquakes and volcanoes,similar to living along the Ring of Fire on Earth. However, I'm not really familiar enough with geology and plate tectonics and the like to know if this would actually work.

As I said, I am not sure this set up actually can be made totally realistic, but I at least want to do what I can, and please feel free to point out any other issues I maybe neglected to consider.


  • $\begingroup$ Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. $\endgroup$
    – Community Bot
    May 9 at 19:02
  • $\begingroup$ Rock formations move because they're under such extreme pressures that the rock itself can plastically deform. These conditions are quite thoroughly incompatible with the existence of caves. This is why Project Iceworm (en.wikipedia.org/wiki/Project_Iceworm) was abandoned after just a few years, and it sounds like you have something even less stable in mind. $\endgroup$ May 9 at 20:10
  • $\begingroup$ A subsurface motion as rapid as you describe would result in a hellscape. What I don't understand is why you need it to be realistic? Per the help center, we focus on helping people create imaginary worlds. Yes, we embrace so-called "real world" questions, but the further you are away from the real world, the less likely the real world can help you. I like the idea. Why is the idea of it not being "realistic" enough stopping you from continuing to build your world? $\endgroup$
    – JBH
    May 9 at 20:24

1 Answer 1


Handwave realism

Think about mixing concrete. The purpose of the mixing process is not only to evenly distribute the materials, but to eliminate air voids that would be structural weaknesses when the concrete sets. It does not matter too much how the mixing is accomplished, but without deliberately injecting air into the concrete the outcome will be to eliminate the air, which will bubble to the surface and be released. The same thing will happen on a planetary scale - heavier things sink below lighter things as they mix around, which is why the atmosphere is above the ocean, which is above the solid crust etc. Geological activities may push magma up into the air at times, and evaporation is constantly putting moisture into the atmosphere, but big, habitable bubbles of breathable air do not go diving down into the ocean or the crust.

"Ah-ha!" I hear you say, "but what about cakes?" Yes, self-raising flour or bicarb soda will result in gas pockets being created through a chemical process. That chemical process will not create breathable air, nor will it continue beyond the point at which the relevant chemicals have undergone their exothermic reactions with each other and achieved a low-energy, stable state.

The other problem with having stable-but-constantly-moving inhabited underground caves is the food web. On Earth, the base of all food chains is some plant using chlorophyll to turn sunlight into usable energy. (They are also needed to prevent the oxygen-breathers suffocating as CO2 builds up.) Something needs to do the same thing in each cave ecology, or there will be no food at the bottom of the pyramid or breathable air, but it needs to do it without sunlight.

Putting this together, the situation you describe with large, habitable caves filled with their own ecologies - including megafauna! - cannot evolve into existence in the first place or continue naturally (without even looking at the secondary effects of the high-churn vulcanism). I see two options here:

  1. As JBH suggests, ignore the realism of the situation and handwave it. Describe effects and observations, but not the causes or scientific rationale. No explanation is better than a lame one.
  2. If it cannot occur rationally or with any known human science, make it an outcome of hypothesised super-alien machinery buried even deeper in the planet that keeps releasing these bubble caves and maybe replenishing them. The machinery is (assumed to be) at the depth of (beyond the ability of the humans to reach or even map). Some human scientists spend lots of time scratching their heads over how it could be possible, but the miners you are writing about are in it to make a living and worry about how to exploit the resources without dying rather than how or why the godlike aliens set this up.
  • $\begingroup$ Thanks guys, yeah thinking more yeah I am honestly getting too hung up on the realism aspect and potential nitpicks and whatever else, and its helpful to realize that. $\endgroup$ May 11 at 13:41

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