[ardecila]

Exactly. Even short tunnels like subways and sewers often run into lots of unforeseen conditions, because we really have a terrible sense of what's going on underground; water movements, changes in soil type, fault lines, gas deposits, and so forth all contribute to uncertainty in tunnel construction.

My sense is that in rural areas, one could use cut and cover techniques to keep the costs predictable, with very shallow tunnels and prefabricated tunnel sections. Even for this, the costs are predictable but not low - digging hundreds of miles of trench and then backfilling is not cheap. It is commonly done for oil pipelines, though not at the diameter needed for transport. In urban areas, you would need to use bored tunnels to avoid huge disruption to the surface, and since our urban areas are so sprawling, that's a lot of deep tunneling with lots of uncertainty.