Underground housing, underground businesses, etc. Would that be better for the environment + possibly save on energy costs? Also possibly safer in certain scenarios like tornadoes etc.
Potential issues that immediately come to mind are ventilation, earthquakes, and flooding. But it’s not like underground dwellings/basements/etc. aren’t a thing, so maybe those issues have been addressed in ways I’m not familiar with.
Mold and humidity, look at people who do that and they have to run a heater and dehumidifier to keep the moisture down. Now you could colocate a nuclear reactor and have built in heating and large scale forced air and that would solve it.