* Digging is expensive. There's several factors for this, but some unavoidable stuff include things like "you need to put the stuff you took out of the ground somewhere."
* The deeper you go, the hotter it gets. Although it's probably not that bad if you're only digging a mile or so.
* Natural light is hard to come by underground.
* Groundwater intrusion becomes a much more significant concern.
* Having to pump all of the sump water and sewage up to the sewer lines is not going to be fun.
Several issues:
* Digging is expensive. There's several factors for this, but some unavoidable stuff include things like "you need to put the stuff you took out of the ground somewhere."
* The deeper you go, the hotter it gets. Although it's probably not that bad if you're only digging a mile or so.
* Natural light is hard to come by underground.
* Groundwater intrusion becomes a much more significant concern.
* Having to pump all of the sump water and sewage up to the sewer lines is not going to be fun.