> Life in a Martian colony would be miserable, with people forced to live in artificially lit underground bases, or in thickly protected surface stations with severely minimized access to the outdoors. Life in this closed environment, with limited access to the surface, could result in other health issues related to exclusive indoor living, such as depression, boredom from lack of stimulus, an inability to concentrate, poor eyesight, and high blood pressure—not to mention a complete disconnect from nature.

I don't know why there's this assumption that living underground has to be in some dank basement with flickering fluorescent tubes, and that this is an absolute unsolvable barrier to Mars settlement.

I think we should just accept that if humans go to Mars, they're going to live underground and either go outside rarely or not at all, and just get over it. Maybe it won't be all that bad if it's done the right way. Realistically, a lot of space is going to be needed just for growing food, and that means the population per square foot of usable pressurized habitat floor space might not actually be very high. There may be wide open spaces and the opportunity to go on long walks through pleasant farms.

In Oregon near Bend we have a pretty decently large lava tube that's about a mile long, and in some parts it's huge. It's not exactly a pleasant place to imagine living when it's cold inside and lit by flashlight or latern, with walls that look like the inside of a dragon's colon and a floor of featureless sand that plays tricks with depth perception, but if one imagines it well lit with sealed, insulated walls, painted in light colors, or made to mimic the sky, with clusters of apartments and workshops and rows of plants, it actually could be pretty nice. The weirdest thing to get used to would be the one-dimensionality of the cave; it's just one long shaft with no side tunnels. That's what I imagine an early Mars habitat looking like: find a natural lava tube (due to reduced gravity they should in theory be bigger than the ones on Earth), and seal the walls and divide it into large compartments. Install solar panels on the surface, and then move right in.

Eventually one could imagine a habitat expanding by digging side tunnels once the right kind of heavy equipment is available, and maybe establish production for native building materials like sulfur-based concrete.