Tunnels of this conformation (geometrically straight and therefore dipping into and through crust and mantle) have been semi-seriously proposed as transportation systems. They have the advantage that (according to math I can't claim to understand) gravitationally driven, unpowered travel through such a tunnel (evacuated, with frictionless rails) would take a constant time for any transit, regardless of end points.

For a tunnel open at the ends, the most salient issue would be atmospheric pressure. You calculation shows 271 km depth at the midpoint -- that's more than four times the depth of the troposphere on Venus, indicating that you'll have a pressure near 400 atm at the midpoint (a bit less, perhaps, because Earth's gas mixture is less dense than the cardon dioxide that makes up most of the Venerian atmosphere). That's enough pressure to make the tunnel impassible to current travel technology -- we don't even have submarines that are self-powered that can stand the equivalent of 40 km depth of sea water. And then there's temperature -- not due to heat flux through the (perfectly insulative) tunnel wall, but due to simple pressure/temperature relationships. The same lapse rate that makes it cooler in the mountains than at the beach will make it hellishly hot by the time you're a few tens of kilometers into the tunnel (and a kilometer or so below sea level).

Further, since the air in the tunnel will interchange with the air outside, and the size of the tunnel (far larger than airship hangars, the overall largest enclosed structures), there will be weather inside the tunnel. Lack of solar heating suggests that much of this weather will involve deposition of water introduced with inflowing air (rain, fog, condensation on the walls). Additionally, air can hold less water vapor at higher pressure. So there'll be water inside the tunnel, and we literally have no reliable idea what happens to ambient temperature water at 400 atm. Too hot to make pressure ice, I think, quite possibly supercritical (or near critical -- read Close to Critical by Hal Clement for how that would act).

A kilometer of rain? Impossible! But it won't take a kilometer of rainfall to close the tunnel; it will take a small fraction of that amount, because the midpoint is gravitationally "down" from all other points in the tunnel, so all the rainfall within the tunnel (as well as "natural" rain that fall into either opening) will run down to the center. In a way, it won't matter; no human will ever see the midpoint of the tunnel.

Tunnels of this conformation (geometrically straight and therefore dipping into and through crust and mantle) have been semi-seriously proposed as transportation systems. They have the advantage that (according to math I can't claim to understand) gravitationally driven, unpowered travel through such a tunnel (evacuated, with frictionless rails) would take a constant time for any transit, regardless of end points.

For a tunnel open at the ends, the most salient issue would be atmospheric pressure. You calculation shows 271 km depth at the midpoint -- that's more than four times the depth of the troposphere on Venus, indicating that you'll have a pressure near 400 atm at the midpoint (a bit less, perhaps, because Earth's gas mixture is less dense than the cardon dioxide that makes up most of the Venerian atmosphere). That's enough pressure to make the tunnel impassible to current travel technology -- we don't even have submarines that are self-powered that can stand the equivalent of 40 km depth of sea water. And then there's temperature -- not due to heat flux through the (perfectly insulative) tunnel wall, but due to simple pressure/temperature relationships. The same lapse rate that makes it cooler in the mountains than at the beach will make it hellishly hot by the time you're a few tens of kilometers into the tunnel (and a kilometer or so below sea level).

Further, since the air in the tunnel will interchange with the air outside, and the size of the tunnel (far larger than airship hangars, the overall largest enclosed structures), there will be weather inside the tunnel. Lack of solar heating suggests that much of this weather will involve deposition of water introduced with inflowing air (rain, fog, condensation on the walls). Additionally, air can hold less water vapor at higher pressure. So there'll be water inside the tunnel, and we literally have no reliable idea what happens to ambient temperature water at 400 atm. Too hot to make pressure ice, I think, quite possibly supercritical (or near critical -- read Close to Critical by Hal Clement for how that would act).

A kilometer of rain? Impossible! But it won't take a kilometer of rainfall to close the tunnel; it will take a small fraction of that amount, because the midpoint is gravitationally "down" from all other points in the tunnel, so all the rainfall within the tunnel (as well as "natural" rain that fall into either opening) will run down to the center. In a way, it won't matter; no human will ever see the midpoint of the tunnel.

To be perfectly clear: this tunnel is not a horizontal tube. Horizontal would follow the Earth's reference datum (sea level, which is very nearly spherical; its oblateness due to Earth's rotation doesn't matter here); you can clearly see from the diagrams in the question that, relative to the surface at either end (presumed level ground) the tunnel slopes by several degrees -- and if it slopes down toward the midpoint at both ends, the midpoint must be the bottom. Don't let the geometric straightness of the tunnel fool you; the midpoint is genuinely 271 km below sea level as it would exist directly above the midpoint.

Tunnels of this conformation (geometrically straight and therefore dipping into and through crust and mantle) have been semi-seriously proposed as transportation systems. They have the advantage that (according to math I can't claim to understand) gravitationally driven, unpowered travel through such a tunnel (evacuated, with frictionless rails) would take a constant time for any transit, regardless of end points.

For a tunnel open at the ends, the most salient issue would be atmospheric pressure. You calculation shows 271 km depth at the midpoint -- that will give (guessing, because there are multiple variables in play) 15-30 atmospheres ambient pressure at that lowest point. That's enough pressure to make the tunnel impassible to current travel technology (we have vehicles that can take that much pressure under water, but they don't have wheels or wings). And then there's temperature -- not due to heat flux through the (perfectly insulative) tunnel wall, but due to simple pressure/temperature relationships. The same lapse rate that makes it cooler in the mountains than at the beach will make it hellishly hot by the time you're a few tens of kilometers into the tunnel (and a kilometer or so below sea level).

Further, since the air in the tunnel will interchange with the air outside, and the size of the tunnel (far larger than airship hangars, the overall largest enclosed structures), there will be weather inside the tunnel. Lack of solar heating suggests that much of this weather will involve deposition of water introduced with inflowing air (rain, fog, condensation on the walls). That, in turn, suggests that the tunnel will be impassable, closed by rising water, before it has existed for too long a time.

A kilometer of rain? Impossible! But it won't take a kilometer of rainfall to close the tunnel; it will take a small fraction of that amount, because the midpoint is gravitationally "down" from all other points in the tunnel, so all the rainfall within the tunnel (as well as "natural" rain that fall into either opening) will run down to the center. I'd guess (without any calculations), based on average equatorial African conditions, that it might take as little as a decade for the midpoint of the tunnel to required boats to pass, and less than century to require submarines.

Tunnels of this conformation (geometrically straight and therefore dipping into and through crust and mantle) have been semi-seriously proposed as transportation systems. They have the advantage that (according to math I can't claim to understand) gravitationally driven, unpowered travel through such a tunnel (evacuated, with frictionless rails) would take a constant time for any transit, regardless of end points.

For a tunnel open at the ends, the most salient issue would be atmospheric pressure. You calculation shows 271 km depth at the midpoint -- that's more than four times the depth of the troposphere on Venus, indicating that you'll have a pressure near 400 atm at the midpoint (a bit less, perhaps, because Earth's gas mixture is less dense than the cardon dioxide that makes up most of the Venerian atmosphere). That's enough pressure to make the tunnel impassible to current travel technology -- we don't even have submarines that are self-powered that can stand the equivalent of 40 km depth of sea water. And then there's temperature -- not due to heat flux through the (perfectly insulative) tunnel wall, but due to simple pressure/temperature relationships. The same lapse rate that makes it cooler in the mountains than at the beach will make it hellishly hot by the time you're a few tens of kilometers into the tunnel (and a kilometer or so below sea level).

Further, since the air in the tunnel will interchange with the air outside, and the size of the tunnel (far larger than airship hangars, the overall largest enclosed structures), there will be weather inside the tunnel. Lack of solar heating suggests that much of this weather will involve deposition of water introduced with inflowing air (rain, fog, condensation on the walls). Additionally, air can hold less water vapor at higher pressure. So there'll be water inside the tunnel, and we literally have no reliable idea what happens to ambient temperature water at 400 atm. Too hot to make pressure ice, I think, quite possibly supercritical (or near critical -- read Close to Critical by Hal Clement for how that would act).

A kilometer of rain? Impossible! But it won't take a kilometer of rainfall to close the tunnel; it will take a small fraction of that amount, because the midpoint is gravitationally "down" from all other points in the tunnel, so all the rainfall within the tunnel (as well as "natural" rain that fall into either opening) will run down to the center. In a way, it won't matter; no human will ever see the midpoint of the tunnel.