Honestly, I kind of wish that underground megastructures were more of a thing here on Earth. Here, you'd run into trouble with groundwater, but that should be

Honestly, I kind of wish that underground megastructures were more of a thing here on Earth. Here, you'd run into trouble with groundwater, but that should be solvable. A bigger objection is that natural caves really ought to stay natural so that people can enjoy them or animals can use them as habitat. Filling in man-made quarries and pit mines with human structures could be done, though. I live in a city that has a giant hole in the ground that I never knew about until I drove past it one day; I fantasize about building a massive apartment complex / shopping center / park there, but am prevented from doing so because a) I don't have even a tiny fraction of the money required for such a project and b) the city land use planning department (which happens to be next door to the giant hole) would never allow me to do that even if I could afford it. Anyways, part of the interest in living in a habitat on Mars is that it would be on Mars, doing (potentially) important work establishing an outpost in space that gives us easier access to the rest of the solar system. Rocket fuel can be made on Mars, and the weak gravity makes it easier to transport stuff up and down the gravity well. Getting back and forth to the belt (which may have significant mineral resources) is easier than from Earth. Basically, going to Mars isn't about just going to Mars, it's about establishing a foothold in space. It's also something we could plausibly do with current technology, if there's enough funding and enthusiasm to make it happen.