There is the small matter of geology, following have to be considered, for example: -Rock type -Groundwater flow -Tectonic activity You can have a whole car

There is the small matter of geology, following have to be considered, for example: -Rock type -Groundwater flow -Tectonic activity You can have a whole career as a geotechnical engineer. There are genuine and difficult problems that require consideration if your tunnel isn't to collapse after a few years of use. I don't think these problems magically disappear with new technology or going deep enough.