Comment by yourapostasy

> ...boring through rock is the most expensive way to connect 2 places.

I get the impression Musk looked at that expense, brought in some subject matter experts, probed them on why that was the case, and decided there must be a better way. From what I've read of tunnel boring, I can see that happening. Not uncommon for a tunnel boring machine to be custom-built and then abandoned in place at the end of the project. A more general-purpose or modular machine might be what he's thinking about. Unexpected geologic formations come up frequently, a function of the expense of manually-intensive sampling processes today. A robotic sampling crawler that bores hundreds of samples, and automatically classifies as it goes instead of sending to an off-site lab, building a 3D map as it moves, might be what he's looking into.

There might be red tape issues even if these technical challenges are overcome, though. Even if you propose going so deep you avoid all other underground infrastructure altogether, government bureaucracies' permitting and related processes will put a floor on the cost avoidance Musk seeks.