If they are large enough, caverns will feel like the outdoors; they might even be plumbed for "rain" and specially vented to create "wind." Artificial weather will keep the air crisp, while artificial light sources - from vast LED arrays, fiber pipes carrying light from the surface, genetically engineered extra-phosphorescent lichen - will infuse this superspace in an eternal dawn. Sunbathers, though, will need to call for the elevator.
And, as always, artists are the shock troops of gentrification:
If history is any guide, artists will be the first to actually move underground full-time. They have a knack for converting industrial and commercial spaces into highly desirable residential real estate. Looking at their airy studios, we'll decide underground space isn't so dreary after all.
<SPECULATION> I wonder how the economics will work; will above-ground real estate become a highly-prized status symbol for the ultra-rich? Or perhaps pollution, ozone depletion or other catastrophes (nuclear fallout perhaps?) will make "up top" into a slum inhabited only by outlaws and untouchables; and as the mole people tuck their children into their beds belowground, they'll tell them blood-curdling tales about the monsters and fiends who live on the surface (much as have been told throughout the aeons about any desolate wastelands outlaws take refuge in). </SPECULATION>
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