Underground is not necessarily better for the environment. I think if we compared the ideal underground build to the ideal above ground build, underground would actually be worse for the environment.

Think about it this way: The advantages you might get from underground are related to reclaiming ground and comprehensive city planning. But you can reclaim roof space to make up for the ground, and you can get the same benefits from city planning building above ground.

The idea that you’d just leave pure wilderness on the ground level when you build underground is not realistic. You could grow the crops you need right there above it, instead, for example. A certain amount of land is needed to support each person. But either way, people would be going to the surface every day. If you build underground, you’ll also be building above ground.

Meanwhile, underground requires quite a bit more stuff. You have to plan more to manage heat and ventilation. It’s difficult to increase density underground because you can build higher more easily than you can dig deeper.