> There’s also lots of room underground to build vs surface, so little constraint there. There are lots of constraints building underground. Because on ground

> There’s also lots of room underground to build vs surface, so little constraint there. There are lots of constraints building underground. Because on ground level the buildings have been built with certain assumptions about the ground regarding the construction structure. When the underground now changes, then either the ground will collapse or the underground tunnels have to been build statically according to those requirements. That is the hard work of tunneling. Also maintaining the tunnels regarding ground water levels is hard. Changing ground water levels also has impact in the statics of the building on the ground. There are much more things to consider building and maintaining underground.