Comment by ProblemFactory

Is technology really the slow and expensive part of boring tunnels? To me it has always seemed that tunnels are expensive because:

* Planning the traffic impact and how it interacts with other transport lanes, all the above-ground building work to make the tunnel useful,

* Emergency ventilation and escape routes, and all other infrastructure that goes into the hole,

* Avoiding any building foundation piles, unmapped tunnels, pipes and cables, re-planning and fixing them if you do hit them,

* In Europe at least, archeological surveys.