> Solution: Do not connect the tunnel directly to the road network.

Then how do people get there? Train? OK, then you have to plan how it interacts with the train network's traffic and schedules, duh.

At least that's easier than car traffic... OH WAIT! Train stations usually have parking, or even if they didn't, are in the middle of a city, not at the top of a tower in the himalayas. So even if you don't want them to, people can drive to near an entrance and park. There will be induced traffic, and there is no way to keep the two networks unrelated without making the tunnel network totally useless.

> Solution: Build a kind of infrastructure that only allows onboard backup as emergency ventilation.

FWIW This also means you have to ban internal combustion engines (which should be OK). Just to deal with ICE exhaust, the Holland tunnel replaces all the air inside it every 90 seconds.

> Solution: Minimize "all other" which is not simply the hole.

You don't even know what "all other" is, do you? I don't either, but this whole set of solutions is really borderline "Why don't they just make the whole plane out of the black box?" ignorant sounding.

> Solution: Go deep enough to avoid all of these.

And I'm as ignorant as you, but I remember reading somewhere that digging costs can be exponentialish as depth increases, for a bunch of reasons.

Your reasoning applies equally to airports. Congrats, you just "debunked" the airplane! (duh!)

So even if you don't want them to, people can drive to near an entrance and park. There will be induced traffic

This is vacuous. There will always be induced traffic for such a project. But the kind of planning required for the kind produced by an airport is materially different from the kind produced by an entirely new highway. Also, if your new "airport" requires the construction of a new road in the future, this isn't as big a short term barrier as those around an entirely new major highway, and will be borne by the local government.

FWIW This also means you have to ban internal combustion engines

FWIW, I'm glad you finally caught up with my actual idea!

You don't even know what "all other" is, do you?

That's not relevant at this level. I don't have to know all the details of building a road to know that not building a road and flying a cargo will avoid those expenses and incur different ones. What is your point, here?

(Here's a hint, when discussing major infrastructure, don't automatically ascribe to others straw man positions of zero cost! That's not exactly a "charitable" reading.)

And I'm as ignorant as you, but

...a little soft in the reasoning. If digging costs are exponential with depth, then this would be an interesting point. I'm not sure if I should really credit that. Well, I guess one would deserve credit for bringing that to attention. So your comment makes, charitably, a "half point."

That's not really in dispute.

> Congrats, you just "debunked" the airplane! (duh!)

If by “debunked the airplane” you mean “demonstrated the already widely-accepted fact that air travel only is effective with considerable attention to the surface transport connecting airports to the places people actually want to go to and from in the area serviced by the airport”, sure.

Well, yes! Perhaps I shouldn't have used the click-baity, "Solved:" but those are my best guesses as to the strategies used to minimize the problems.

-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.