How far underground could a human civilization be before it is unsustainable/inhospitable?

For a civilization like this, firstly, I would like to know what dangers there are, e.g: radiation, cave-ins, temperature. I also want to know about the sustainability of this colony, power I'd imagine isn't a issue (geothermal), but how would there be a sustainable source of food? What about mental affects, I'd imagine that the whole lack of sunlight could cause chaos for the brain, but maybe there are other side-effects on being underground, that would cause problems in the brain.

Finally, factoring in all the dangers and requirements of a colony, could this colony even exist (don't worry about how they got down there), and how far underground could this colony even be? The technology is roughly current day, maybe a little bit further into the future, and the colony size is whatever the situation you come up with can support.

Bonus Question: For the chosen depth, what would the geology be? Would it primarily be rock or quartz? What ores would there be, and in what quantities (roughly)?

posted over 9 years ago

Orfby‭

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