It has occurred to me, that there exists the possibility that complex and intelligent life may be present on planetary bodies in our solar system and elsewhere, under circumstances which would cause it to be difficult if not impossible to detect.

Perhaps it is better said that I believe that the possibility itself is possible - and that this thread is an attempt to determine to what extent it is so, through the application of thought and discourse that only the type of intelligent minds that inhabit these forums are capable of.

Here on earth, we human beings take shelter underground in the face of all manner of disasters, natural and otherwise. From tornadoes to thermal nuclear fallout, the protections offered by being underground are well established. It takes only 3 feet of dirt for example to protect against the amount of radiation expected from a range of events, and more complex living environments are within the range of even our present level of technology.

The scenario then is as follows. There are a few environments in our solar system which are currently uninhabitable, but that may have been otherwise previously. At least one of these might have been habitable in the distant past - Mars. While the available facts would seem to rule out the past presence of any large civilization, as evidenced by the lack of structural artifacts one would expect such a population to leave behind...absence of evidence is not evidence of absence - as Dr. Sagan wisely said.

The failure to find ruins of civilization on another planet aside, and leaving untouched for the sake of this discussion the rather large question of whether the planet was habitable long enough for intelligent, complex life to develop sufficiently...an interesting concept presents itself when considering this idea. That a doomed civilization, lacking advanced capabilities in space travel and related technologies, might have an option which is far lower on the technology scale, though admittedly still considerably demanding in that respect. That being, taking refuge underground.

I should say, I am not suggesting that there are actually beings living underground on the planet Mars. I am however finding myself realizing that failing to have a means of moving people off-planet in the face of some looming threat, would not necessarily mean that population would be doomed. Obviously even with reasonably advanced space travel, 'saving' a large portion of a planet's population is most likely impossible - for starters, you need a place to move them to - such is the case as well for a refuge underground. But it would not require unrealistically large numbers of people in order to preserve the society, albeit in a crude and reduced form.

So, in such a scenario, what level of technological sophistication must a society possess in order to exist indefinitely underground? What specific technologies and capabilities would they need to have? How much time would be needed to put this knowledge into action on the scale needed to accommodate a good-sized population, and in the individual case of Mars, would this amount of warning had been available? Would a Martian society of this minimum level of sophistication have been able to predict, and respond to, the changes in their planetary environment quickly enough?

These are questions that I think we can answer, at least in a general way. Thoughts?

In order to keep food a food cycle you need energy. geothermal powerplants arent too dificult to do. but you need at least to have passed into the industrial age.

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Geothermal power is certainly a suitable technology, and not requiring too advanced a civilization - though the general view is that there is no significant geothermal activity on Mars today.

This however is not conclusive, and even if it were, it would not be a problem in the context of this scenario, as we are looking at a time more distant in the past of Mars in any case...or, in a general theoretical sense, any planet facing some kind of event which would drive them underground.

Food/pwer then is obviously a concern, potentially addressable. Another issue I see is that of the other consumables - air (or whatever gas these people would breath), and water. Of course if you have water you can produce air, so this boils downs to - pardon the pun - a need for accessible water. In the case of Mars, it is believed there is subsurface water/water ice present, so this too would seem to be resolvable.

What level of technology then, would be required to produce oxygen from water?

Trees!

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I would think that first such a civilization would have been on the surface with a higher technology than current human civilization. Then they would have developed beyond our technology to engineer this subterranean civilization. But then we'd have seen a lot of their artifacts they left behind on the surface with Mars Rovers, etc.

There is a problem with the idea of building a civilisation under the surface of a geologically inactive world like Mars. If you extract subterranean heat energy from a place where it is not being actively replenished, all you will do is cool the local rock; your heat extraction equipment gets surrounded by a zone of relatively cooler rock, so you have to drill another borehole.

This leads to diminishing returns; after a while, you would spend all the energy you extract from the deep rock drilling new boreholes. In short the energy returned on energy invested would fall to 1:1, and your civilisation would cease to operate.

To make a geothermal civilisation work, you need a geologically active world, like the Earth (althugh other sources of energy are currently more convenient here) othewise you could try the Galilean satelites (especially Io), or Saturn's Enceladus, or anywhere where tidal action causes geological activity.

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You could also harness the heat produced by natural radioactivity, for which you don't need a geologically active world. Of course, whether or not you could get enough energy this way to sustain a civilization is questionable.

I'm reminded of Freeman Dyson's speculations about the survival of civilisation into the very distant future. Once all the stars have gone out, there will be very little free energy left to run a civilisation- so to survive, a distant future society would need to slow down, taking days to do what had taken seconds before.

The same could apply to a civilisation trying to live off a tiny underground heat gradient- it could live slowly, using the tiny amounts of available energy to process data and construct infrastructure. The complexity of a civilisation is a separate measure to its speed.

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Most of the posts, including the OP are good analysis. Why wait for the disaster? We could incarcerate selected criminals underground now. Left alone in small groups, with no promise of the arrival of supplies from the surface, some of these people would think hard about self sufficiency, and likely achieve some breakthroughs, if we are generous with lab equipment, but not food, water and oxygen.
The number needed to succeed long term, depends a lot on the work habits and intelligence of the subjects, and how they dispose of those who are not part of the solution. A major disaster for Earth, might leave the criminals far below the surface the only survivors, so their route to the surfaces should unlock automatically monthly unless reset by the persons delivering marginal supplies.
It tends to get very hot a mile or more below Earth's surface, due to the waste heat produced by even a tiny underground colony. I think this is true of even tiny asteroids and comets whose outer surface are at perhaps 15 degrees k.
While there are likely few bodies in our solar system with centers as hot as Earth, my guess is few are as cold as 100 degrees k at the center. Mars could easily have liquid water 20 plus miles below the surface. The decay of isotopes alone likely makes the center of Mars 293 degrees k or warmer. The heat of the colony can produce some energy as it is sent closer to the surface where it is considerably colder. Neil

This, to me, sounds rather cruel and unusual. I could not support the sort of system that would choose this over rehabilitation.

But, as a whole, perhaps we won't consider geothermal energy until our current, more convenient sources of power dwindle away.

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geothermal energy is already in use in areas where it is easily accessible and plentiful at the surface. like Iceland.

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