Shar_Lamagne wrote:What about permanent underground cities? Would it be possible to construct underground self-sustaining biospheres?
Do you mean here on Earth?
In H.G. Wells' Time Machine they... Oh wait, that's science fiction again. Damn. Seems we need a real world example of self-sustaining biospheres. What ever happened with that experiment they were doing in the '90's? I don't remember that being a resounding success. And those were above ground.
To be a little more serious, can you imagine the amount of electricity we would need to grow crops underground? Geothermal source, right?